Add skill to trigger ADO pipelines in PRs (microsoft#26632)

## Description

Adds an agent skill to trigger all pipelines in a PR. Useful for
Copilot-generated PRs where a repo maintainer needs to manually trigger
the pipelines. The comment bodies are taken from our internal docs. If
we merge this, we probably want to update those docs to point to the
skill so we have a single source of truth.

## Reviewer Guidance

The review process is outlined on [this wiki
page](https://github.com/microsoft/FluidFramework/wiki/PR-Guidelines#guidelines).

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: alexvy86

.claude/skills/trigger-pipelines-for-copilot-pr/SKILL.md

@@ -0,0 +1,42 @@
+---
+name: trigger-pipelines-for-copilot-pr
+description: Trigger ADO pipelines for a Copilot-created PR by posting /azp run comments. Use when the user asks to trigger CI pipelines for a specific PR.
+allowed-tools: Bash(gh pr comment *)
+context: fork
+model: claude-haiku-4-5-20251001
+argument-hint: [pr-number-or-url]
+---
+
+Post the following two comments to the PR specified by the user ($ARGUMENTS), in the `microsoft/FluidFramework` repository on GitHub.
+They need to be posted separately because otherwise it gets too long and
+fails to trigger correctly.
+
+First comment:
+
+```
+/azp run Build - protocol-definitions,Build - test-tools,server-gitrest,server-gitssh,server-historian,server-routerlicious,Build - client packages,repo-policy-check
+```
+
+Second comment:
+
+```
+/azp run Build - api-markdown-documenter,Build - benchmark-tool,Build - build-common,Build - build-tools,Build - common-utils,Build - eslint-config-fluid,Build - eslint-plugin-fluid
+```
+
+Posting those comments will trigger all our pipelines, which is necessary for PRs that are created by Copilot.
+
+To post the comments first check if the GitHub CLI is available,
+and if so use `gh pr comment <PULL_REQUEST_NUMBER> --repo microsoft/FluidFramework --body "<COMMENT_TEXT>"`.
+If `gh` is not available but `$GITHUB_TOKEN` is, you can try the GitHub REST API directly, e.g.:
+
+```
+curl -L \
+  -X POST \
+  -H "Accept: application/vnd.github+json" \
+  -H "Authorization: Bearer $GITHUB_TOKEN" \
+  -H "X-GitHub-Api-Version: 2022-11-28" \
+  https://api.github.com/repos/microsoft/FluidFramework/issues/PULL_REQUEST_NUMBER/comments \
+  -d '{"body":"<COMMENT_TEXT>"}'
+```
+
+If neither is available, don't do anything and tell the user you can't complete the request and why.