feat(link-backport-prs): link backport PRs to matching Linear sub-issue

.github/actions/link-backport-prs/README.md

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+# Link Backport PRs to Linear
+
+A GitHub Action that links sorenlouv-created backport pull requests to the matching Linear sub-issue, so each backport closes the right per-release-line issue when it merges.
+
+## What it does
+
+When a merged source PR carries `backport-to-<branch>` labels, the [sorenlouv backport action](https://github.com/sorenlouv/backport-github-action) opens one backport PR per label. This action runs right after and, for each backport target:
+
+1. Resolves the source PR's Linear issue (the parent) via Linear's `attachmentsForURL` reverse lookup, falling back to a `TEAM-123` identifier parsed from the branch name or body.
+2. Finds the sub-issue whose title carries the release-line prefix for that target, e.g. `[0.34] Copy of ENGCP-906` for a backport to `v0.34` (a leading `v`, as in `[v0.34]`, is also accepted).
+3. Appends `Fixes <sub-issue-id>` to that backport PR's body, unless it already references the issue.
+
+The match is by title prefix, not milestone: the `[X.Y] Copy of ...` sub-issues created for a backport family do not reliably carry a patch milestone, so the title is the dependable key.
+
+It is advisory and idempotent: it never fails the backport job (every error is a warning and it exits 0), it skips entirely when no `linear-token` is provided, and re-runs do not add duplicate `Fixes` lines.
+
+## Usage
+
+This action is wired into the shared [`backport.yaml`](../../workflows/backport.yaml) reusable workflow. A caller enables linking by passing a Linear token:
+
+```yaml
+jobs:
+  backport:
+    uses: loft-sh/github-actions/.github/workflows/backport.yaml@backport/v1
+    secrets:
+      gh-access-token: ${{ secrets.GH_ACCESS_TOKEN }}
+      linear-token: ${{ secrets.LINEAR_API_TOKEN }}
+```
+
+To run it directly:
+
+```yaml
+- uses: loft-sh/github-actions/.github/actions/link-backport-prs@link-backport-prs/v1
+  with:
+    source-pr: ${{ github.event.pull_request.number }}
+    repo-owner: ${{ github.repository_owner }}
+    repo-name: ${{ github.event.repository.name }}
+    github-token: ${{ secrets.GH_ACCESS_TOKEN }}
+    linear-token: ${{ secrets.LINEAR_API_TOKEN }}
+```
+
+The `github-token` must be the same PAT that created the backport PRs (a PAT, not the default `GITHUB_TOKEN`, so the backport PRs exist and are editable).
+
+## Inputs
+
+<!-- AUTO-DOC-INPUT:START - Do not remove or modify this section -->
+
+|    INPUT     |  TYPE  | REQUIRED |     DEFAULT      |                                                                                                                                  DESCRIPTION                                                                                                                                  |
+|--------------|--------|----------|------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|   dry-run    | string |  false   |    `"false"`     |                                                                                                                   Log intended edits without applying them                                                                                                                    |
+| github-token | string |   true   |                  |                                                                             GitHub token with permission to read <br>and edit pull requests (must be the same PAT that created the backport PRs)                                                                              |
+| label-prefix | string |  false   | `"backport-to-"` |                                                                                                              Prefix of the backport labels on <br>the source PR                                                                                                               |
+| linear-token | string |  false   |                  | Linear API token for resolving the <br>issue family. Optional by design: this <br>is an advisory step, so when <br>empty the action no-ops and exits <br>0 instead of failing, letting callers <br>adopt the shared backport workflow before <br>a Linear token is wired up.  |
+|  repo-name   | string |   true   |                  |                                                                                                                          The name of the repository                                                                                                                           |
+|  repo-owner  | string |   true   |                  |                                                                                                                          The owner of the repository                                                                                                                          |
+|  source-pr   | string |   true   |                  |                                                                                                        The merged source pull request number <br>that was backported                                                                                                          |
+
+<!-- AUTO-DOC-INPUT:END -->
+
+## Outputs
+
+<!-- AUTO-DOC-OUTPUT:START - Do not remove or modify this section -->
+No outputs.
+<!-- AUTO-DOC-OUTPUT:END -->
+
+## Development
+
+### Testing
+
+Run the unit tests:
+
+```bash
+./test.sh
+# or
+make test-link-backport-prs
+```
+
+The tests cover the pure matching logic: release-line extraction from a target branch, title-prefix matching (`[0.34]` and `[v0.34]`), sub-issue selection within an issue family, idempotency of the `Fixes` line, and identifier extraction fallback.

.github/actions/link-backport-prs/action.yml

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+name: 'Link Backport PRs to Linear'
+description: 'Links sorenlouv-created backport PRs to the matching Linear sub-issue by adding "Fixes <id>" to the backport PR body'
+
+inputs:
+  source-pr:
+    description: 'The merged source pull request number that was backported'
+    required: true
+  repo-owner:
+    description: 'The owner of the repository'
+    required: true
+  repo-name:
+    description: 'The name of the repository'
+    required: true
+  github-token:
+    description: 'GitHub token with permission to read and edit pull requests (must be the same PAT that created the backport PRs)'
+    required: true
+  linear-token:
+    description: 'Linear API token for resolving the issue family. Optional by design: this is an advisory step, so when empty the action no-ops and exits 0 instead of failing, letting callers adopt the shared backport workflow before a Linear token is wired up.'
+    required: false
+    default: ''
+  label-prefix:
+    description: 'Prefix of the backport labels on the source PR'
+    required: false
+    default: 'backport-to-'
+  dry-run:
+    description: 'Log intended edits without applying them'
+    required: false
+    default: 'false'
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Set up Go
+      uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
+      with:
+        go-version-file: ${{ github.action_path }}/src/go.mod
+
+    - name: Build and run Link Backport PRs
+      shell: bash
+      env:
+        GITHUB_TOKEN: ${{ inputs.github-token }}
+        LINEAR_TOKEN: ${{ inputs.linear-token }}
+        ACTION_PATH: ${{ github.action_path }}
+        INPUT_SOURCE_PR: ${{ inputs.source-pr }}
+        INPUT_REPO_OWNER: ${{ inputs.repo-owner }}
+        INPUT_REPO_NAME: ${{ inputs.repo-name }}
+        INPUT_LABEL_PREFIX: ${{ inputs.label-prefix }}
+        INPUT_DRY_RUN: ${{ inputs.dry-run }}
+      run: |
+        cd "$ACTION_PATH/src"
+        go build -o link-backport-prs .
+        ./link-backport-prs \
+          -source-pr="$INPUT_SOURCE_PR" \
+          -repo-owner="$INPUT_REPO_OWNER" \
+          -repo-name="$INPUT_REPO_NAME" \
+          -label-prefix="$INPUT_LABEL_PREFIX" \
+          -dry-run="$INPUT_DRY_RUN"
+
+branding:
+  icon: 'link'
+  color: 'orange'

.github/actions/link-backport-prs/src/go.mod

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+module github.com/loft-sh/github-actions/link-backport-prs
+
+go 1.26.4
+
+require github.com/google/go-github/v88 v88.0.0
+
+require github.com/google/go-querystring v1.2.0 // indirect

.github/actions/link-backport-prs/src/go.sum

@@ -0,0 +1,7 @@
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/google/go-github/v88 v88.0.0 h1:dZA9IKkPK1eXZj4ypngnpRj5FwdpTv4whix2PrQMP7M=
+github.com/google/go-github/v88 v88.0.0/go.mod h1:rufTDgn2N45wjhukLTyxmvc9nilSp3mr3Rgtt6b1MPw=
+github.com/google/go-querystring v1.2.0 h1:yhqkPbu2/OH+V9BfpCVPZkNmUXhb2gBxJArfhIxNtP0=
+github.com/google/go-querystring v1.2.0/go.mod h1:8IFJqpSRITyJ8QhQ13bmbeMBDfmeEJZD5A0egEOmkqU=