* feat(server): refresh PR state promptly after agent merges via gh CLI
When an agent merges a PR with gh pr merge, the app could take up to
2 minutes to show the merged state because:
- gh pr merge is a remote GitHub operation that does not touch local git refs,
so file watchers never fire
- the GitHub self-heal poll runs on a 2-minute slow interval for settled PRs
- no refresh is triggered when agent tool calls complete
Add a detection path that intercepts completed shell tool calls in the
agent manager and signals the workspace git service when a command may
have changed external state (gh *, git push, npm publish). The git
service then invalidates the GitHub cache and schedules a forced
GitHub-inclusive refresh, so the updated PR state arrives within ~500ms.
The agent manager does not leak cache semantics; it calls a semantic
onWorkspaceStateMayHaveChanged callback and lets the git service decide
what that means internally.
* refactor(server): share workspace refresh merge logic
