Worktrees: host-owned ownership and an archive lifecycle (snapshot, transfer, keep-count)

[Tryanks]

Summary

Move worktree ownership to the host and give archived threads a Codex-style worktree lifecycle: deferred cleanup, snapshot before removal, ownership transfer, keep-count.

Tcode today

• Nothing happens to a worktree on archive. On delete, an optional git worktree remove --force (branch tcode/<id> kept, uncommitted work lost; crates/services/src/worktree.rs).
• Whether a worktree is shared is decided in the UI (worktree_orphaned_by_delete, crates/ui/src/store/mod.rs) and ignores forks, which share the cwd without owning worktree (Removing a deleted thread's worktree also deletes its forks' working directory #517).
• The startup orphan sweep removes directories whose name is not a known session id, undoing "Keep worktree" (Startup orphan sweep force-removes worktrees the user chose to keep #516).

Codex app behaviour

• [desktop] worktree-auto-cleanup-enabled (default on), worktree-keep-count (default 15).
• After archive, cleanup runs after 60 s; unarchive inside the window cancels it.
• If another unarchived thread uses the worktree, ownership moves to it; otherwise the state is committed to refs/codex/snapshots/<id> ("Codex worktree snapshot: …") and the worktree is deleted; it can be restored.
• Count-based cleanup keeps pinned and in-progress threads' worktrees and deletes at most 20 per run.

Proposal

• Host owns ownership. A worktree is in use while any unarchived session's cwd is inside it. The host computes this; archived_worktree_branches and the UI-side check go away.
• On archive finalisation (after the 60 s window from Archive: align with the Codex app (log move, provider-native archive, undo) #529): if another unarchived thread uses the worktree (a fork, or a thread started in an existing worktree), transfer the worktree ownership field to it. Otherwise snapshot uncommitted state to refs/tcode/snapshots/<id>, remove the worktree, keep the tcode/<id> branch.
• Keep-count. Keep the most recent 15 archived worktrees on disk (setting), cleaning older ones with the same snapshot step.
• Unarchive recreates the worktree from the branch and restores the snapshot.
• Delete honours "keep worktree" with an explicit keep record; the orphan sweep treats any session cwd inside ~/.tcode/worktrees, any pending-cleanup entry and any keep record as known.

Related work

guivieiras/tcode has unmerged work that makes shared worktrees common: reusing existing worktrees from the workspace selector (241616d) and named worktrees with a chosen base branch (8556ded). The ownership model above has to cover threads that join an existing worktree.