Add deadlines to git network ops and lock merges per project

TODO.md

@@ -4,8 +4,6 @@
   `CheckResponse` reads open state outside the write transaction and `Respond` updates by id alone (`internal/agents/respond.go:26–34`, `:93`; `internal/store/inbox.go:140`), so two concurrent answers both land, or a withdrawal is overwritten. Condition the update on `state='open'`, check RowsAffected, roll back the batch on conflict and return 409.
 - A Codex RPC can deadlock the reader it waits on
   `drive` calls `Steer` synchronously while `readLoop` pushes notifications into a 256-entry channel (`internal/codex/session.go:186`, `:440–476`, `:598`); if it fills before the response is read, each waits on the other. Route RPC responses separately from notification consumption and give RPCs bounded deadlines, without replacing backpressure with an unbounded queue.
-- Git network operations have no deadline and run under a cross-project lock
-  `exec.Command` has no context (`internal/git/git.go:348–364`) and Merge holds the shared `mergeMu` through fetch (`internal/agents/mergein.go:68`, `:101`), so one stalled remote blocks merges in every project. Add contextual deadlines and non-interactive git/SSH, and serialize the network phase per project while keeping same-project ordering.
 - A malformed file image URL breaks transcript rendering
   `decodeURI` throws on e.g. `file:///tmp/100%.png` (`web/src/markdown.ts:347`), and `renderAll` appends events before mounting the composer (`web/src/task.ts:608–609`), so the rest of the transcript and the composer don't render. Catch the decode error and render the plain fallback link.
 - Stale reload and focus responses overwrite newer state