feat: change-summary opens a historical PR and requests Copilot code review

[Copilot]

The change-summary workflow only wrote a Markdown report to the job summary. It now also opens a real GitHub PR spanning the requested date window and automatically requests a Copilot code review on it.

Changes

• Permissions — elevated from contents: read to contents: write + pull-requests: write
• Step: Generate change summary — extended to:
  • Tee the Markdown report to /tmp/pr-body.md for use as the PR body
  • Emit base_sha (last commit strictly before start_date), head_sha (last commit at/before end_date), and has_commits as step outputs
• Step: Open PR and request Copilot review (new, conditional on has_commits == 'true'):
  1. Force-pushes two ephemeral branches — change-summary/base-{start_date} and change-summary/{start_date}-to-{end_date} — pointing at the discovered SHAs
  2. Opens a PR (base → head) with the generated summary as its description; falls back to looking up an existing PR rather than failing on re-runs
  3. Requests copilot-pull-request-reviewer via POST /pulls/{n}/requested_reviewers; emits a non-fatal ::warning:: if Copilot code review is not enabled on the repository

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