Let the agent set the merge commit title, defaulting to the task title

docs/operations.md

@@ -21,9 +21,11 @@ Schema changes are the `migrations` slice in `internal/store/store.go`: named en
 
 A task ends `merged` or `discarded`; either way its sessions are stopped, its worktree and branch are removed, its `open` inbox items become `dismissed` (resolved by whoever closed it), and its record and events remain. `GET /api/tasks/{id}/unmerged` reports what would be lost or merged: commits on the branch that aren't on the default branch, and files, lines and uncommitted paths that differ from the merge base. The UI shows it in the confirm dialog of both buttons, which are disabled while the task's session is `running` or `queued`.
 
-`POST /api/tasks/{id}/merge-in` (the transcript's **Merge main in** button, labelled with the default branch) brings the default branch into the task branch through the agent: it sends `mergeInPrompt` (`internal/agents/mergein.go`) as a `byre` message to the task's session, or starts a session with it when the task has none. The prompt has the agent run `git merge --no-ff --no-commit`, resolve conflicts, regenerate generated files rather than hand-merging them, check the build and tests, and leave the merge uncommitted; byre's end-of-turn commit concludes it as a merge commit. It is refused (409) while a session is `running` or `queued`.
+`POST /api/tasks/{id}/merge-in` (the transcript's **Merge main in** button, labelled with the default branch) brings the default branch into the task branch through the agent: it sends `mergeInPrompt` (`internal/agents/mergein.go`) as a `byre` message to the task's session, or starts a session with it when the task has none. The prompt has the agent run `git merge --no-ff --no-commit`, resolve conflicts, regenerate generated files rather than hand-merging them, check the build and tests, and leave the merge uncommitted; byre's end-of-turn commit concludes it as a merge commit. The prompt ends by showing the task's current merge title and asking the agent to set a better one with `set_merge_title` if it no longer describes all the work on the branch. It is refused (409) while a session is `running` or `queued`.
 
-`POST /api/tasks/{id}/merge` starts the same turn and records, in memory, that the task is to be merged by the requesting user, who must have a git identity (400 otherwise). When that turn ends normally (not interrupted, not an error), byre squashes the worktree, now containing the default branch, onto it with `git merge-tree --merge-base`, makes one commit with `git commit-tree` authored and committed by the merging user, runs `git merge --ff-only` in the project's checkout (which must have the default branch checked out), then closes the task. The commit message is the task title. Nothing is pushed. When the merge changes nothing, no commit is made. If the squash fails, for example because the default branch moved again during the turn, byre files an error in the inbox and the task stays active. An interrupted or failed turn, or a byre restart during it, leaves the task active without squashing. `POST /api/tasks/{id}/discard` removes everything without merging.
+A task's `merge_title` is the title of the squash commit Merge makes. It is the task title until the agent calls the `set_merge_title` MCP tool (`internal/agents/mergein.go`), which it may do at any point: one line of at most 72 characters (`maxMergeTitle`), stored in `tasks.merge_title` (empty there means "use the title"), recorded as a `task.merge_title` event that the transcript shows as a note. Malformed titles are rejected with a corrective message, and so is a call once the task is closed. The Merge confirm dialog quotes the title the commit will get.
+
+`POST /api/tasks/{id}/merge` starts the same turn and records, in memory, that the task is to be merged by the requesting user, who must have a git identity (400 otherwise). When that turn ends normally (not interrupted, not an error), byre squashes the worktree, now containing the default branch, onto it with `git merge-tree --merge-base`, makes one commit titled with the task's `merge_title` with `git commit-tree`, authored and committed by the merging user with no agent attribution, runs `git merge --ff-only` in the project's checkout (which must have the default branch checked out), then closes the task. Nothing is pushed. When the merge changes nothing, no commit is made. If the squash fails, for example because the default branch moved again during the turn, byre files an error in the inbox and the task stays active. An interrupted or failed turn, or a byre restart during it, leaves the task active without squashing. `POST /api/tasks/{id}/discard` removes everything without merging.
 
 ## Running as a service