Implement backlog grooming agent

[ReidSS]

Capability request: implement backlog grooming

Summary:

• Implements grooming-agent as a backlog analyzer instead of a stub.
• Fetches open issues, applies readiness checks, labels issues that need grooming, and posts a summary.
• Updates README and adds focused grooming summary tests.

Validation:

• npm run typecheck
• npm test
• npm run build

Review note:

• This PR was produced by the self-improvement flow after the dev watcher interrupted final job bookkeeping.