Add writer and follower roles to bridge coordinator

[wch]

This PR lets multiple development workflows share one ai-provider-bridge build safely. The first watch coordinator becomes the sole writer, while later watch coordinators follow its completed generations and relay readiness markers to their own supervisors. This prevents concurrent Positron, Standalone, and RStudio workflows from fighting over the bridge lock or deleting live output.

Changes

• Records watch or one-shot intent in the coordinator lock and resolves writer/follower roles from the live owner's declared mode.
• Publishes token-scoped ready.json state after each complete watch generation so followers cannot accept stale output from an earlier writer session.
• Adds a generation observer that combines filesystem events with polling, validates live output digests, deduplicates observations, and converges to the latest generation when events coalesce.
• Keeps one-shot and clean builds exclusive, with a clear failure before any output is removed when a live coordinator exists.