ci: add automated weekly release workflow for v2

Adds a GitHub Actions workflow that creates v2 patch releases weekly
(Monday 9 AM UTC) when there are unreleased changelog entries. Mirrors
the v3-alpha autorelease pattern but at a slower, more stable cadence.

The workflow:
- Checks for content in the ## [Unreleased] section of the changelog
- Verifies commits touching v2/ or website/ since last tag
- Increments the patch version (v2.11.0 -> v2.11.1)
- Updates version.txt and changelog.mdx
- Creates a git tag and GitHub Release
- Supports manual dispatch with dry_run and force_release options

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>