feat(github): add integrations to keep pull requests up to date

[dan-petty]

Description

Implement end-to-end integrations to automatically keep GitHub pull requests up to date with their base branches (e.g. main or active release branches).

Objectives & Deliverables

1. GitHub Action Workflow (.github/workflows/update-prs.yml):
   • Automatically trigger when changes are pushed to main or release/** branches.
   • Support workflow_dispatch (with base branch and PR number inputs) and PR comment /update / /sync triggers.
   • Gracefully handle conflicts, draft PRs, and mergeability status.
2. Native CLI Command (devops pr update):
   • Provide devops pr update [PR_NUMBER] with --all, --base, --repo, --expected-head-sha, and --dry-run options.
   • Call GitHub's PUT /repos/{owner}/{repo}/pulls/{number}/update-branch endpoint with rate-limited, token-bucket pacing via run_gh.
   • Rich terminal status reporting.
3. FastMCP Agent Tool (pr_update_branch):
   • Expose MCP tool allowing autonomous agents to update PR branches programmatically.
4. Comprehensive Test Suite:
   • Unit and integration tests in tests/test_pr_update.py verifying single PR update, batch --all updates, conflict handling, --dry-run, and MCP tool wrapper.