Add optimistic concurrency to tracker writes

[justguy]

Tracker task: t16
Tracker: llm-tracker
Swimlane: improvements
Priority: p0

Goal

Allow patch-style writes to include expectedRev so callers can tell whether they are writing against stale state instead of being silently merged over.

Current note

Write responses should distinguish stale-write conflicts from accepted merges.

Tracker context

• Created from the live llm-tracker improvements swimlane backlog.
• Keep the tracker row and the GitHub issue aligned as this work evolves.