Hold a task's merge while another task in the project is merging

docs/operations.md

@@ -41,6 +41,8 @@ A task's participants are the users who put a message bubble with their name in
 
 Before starting the turn, Merge runs `git fetch origin <default branch>` and `git merge --ff-only origin/<default branch>` in the checkout, so the agent merges in what the remote has; the request waits for the fetch, and a failure (network, or a checkout with commits the remote lacks) is returned as the request's error. If the push is still refused, for example because someone pushed during the turn, or fails for any other reason, byre files the error in the inbox and the task stays active; clicking Merge again fetches and goes round once more. A refused push leaves the checkout's default branch where it was, since the squash commit was never moved onto it. If the squash fails, for example because the default branch moved again during the turn, byre also files an error and the task stays active. An interrupted or failed turn, an agent process that exits mid-turn, or a session that can't be resumed after a restart clears `merging_by` and leaves the task active without squashing. `POST /api/tasks/{id}/discard` removes everything without merging.
 
+One merge runs per project at a time. The Manager keeps the task holding each project's merge in memory (`mergeHolders` in `internal/agents/manager.go`), from just before the fetch until the merge closes the task, fails, or is abandoned as above, or the task is discarded; on startup it is rebuilt from the active tasks with `merging_by` set. Merge on another task of that project while one is held doesn't fetch or start a turn: it records the request in `tasks.merge_queued_by` and `merge_queued_at` (`merge_queued_at` on the Task), emits `task.merge_queued` (a transcript note), and returns the task with 202. A second Merge on a held or merging task is a 409. Whenever a project's merge is released, any task turn in it ends, and on startup, byre starts the oldest held merge in the project whose task session is not `running` or `queued`: it clears the request (emitting `task.merge_unqueued`, not drawn) and goes through the same fetch and turn as a direct Merge by the user who asked, so it merges in what the earlier merge pushed. If that start fails (the fetch, or the user has no git identity), byre files the error in the task's inbox and tries the next held merge. The Merge button reads **Merge held**, disabled, while the task has `merge_queued_at`; discarding the task drops the request. `sqlite3 byre.db "SELECT id, merge_queued_by, merge_queued_at FROM tasks WHERE merge_queued_by != ''"` lists held merges.
+
 ## Running as a service
 
 The listen address is `listenAddr` in `cmd/byre/serve.go`; it binds loopback only.