Add workflow to handle README-only PRs

Adds a GitHub Actions workflow that:
- Detects PRs that only modify README.md
- Comments explaining we no longer accept new server additions
- Directs contributors to the MCP registry instead
- Allows updates/removals via /i-promise-this-is-not-a-new-server
- Uses labels (readme: pending / readme: ready for review) for filtering
- Minimizes bot comment after confirmation to reduce clutter

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