🤖 ci: add linux hm activation workflow

What:
- add GitHub Actions workflow for x86_64-linux HM activation check

Why:
- unify HM coverage in CI alongside macOS
- avoid relying on Garnix for test signal

Tests:
- nix build .#checks.x86_64-linux.hm-activation --print-build-logs --accept-flake-config (ok; eval warnings)

Author: DJTBOT

.github/workflows/hm-activation-linux.yml

@@ -0,0 +1,27 @@
+name: HM Activation (Linux)
+
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+  workflow_run:
+    workflows: [ "Yolo Update Pins" ]
+    types: [ completed ]
+
+jobs:
+  hm-activation-linux:
+    if: ${{ github.event_name != 'workflow_run' || (github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.head_branch == 'main') }}
+    runs-on: ubuntu-latest
+    env:
+      TARGET_SHA: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.head_sha || github.sha }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ env.TARGET_SHA }}
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v13
+
+      - name: Run HM activation
+        run: nix build .#checks.x86_64-linux.hm-activation --print-build-logs