[Bug]: PR-status poller retries forever on a worktree folder that is no longer a git repo#24797

What happens

A thread's worktree folder can outlive its git data. The folder stays, empty, with no .git. T3 still polls PR status for that thread. Every run fails with not_a_repository. T3 never stops or marks the worktree missing. The same paths fail roughly every 100 s, for days.

Numbers (one self-hosted server, t3 serve 0.1.3-nightly.20261023.3566)

• 10,309 not_a_repo failures in 41 h on 7 folders, about one per folder every ~100 s.
• Over the server's whole log: 34,924 such failures on the same 7 paths.
• In 110 min of trace before 08:40 CET on 2026-10-25: 6,525 PR checks (p95 5.8 s) and 19,855 git commands. The dead folders made up part of that load and all of the errors.
• All 7 folders were empty directories. Each had been dead for 3 to 5 days.

Sample log line:

Why (source on main, commit 807319c)

• The poller runs every minute for every unsettled thread with a branch: ThreadPullRequestService.ts#L400.
• It checks only that the worktree path exists, then runs git there: ThreadPullRequestService.ts#L188-L197. An empty folder passes fileSystem.exists, so T3 never falls back to project.workspaceRoot.
• The first git call is git remote in that folder: GitManager.ts#L2511-L2518.
• The failure is already classified as not_a_repository: GitVcsDriverCore.ts#L513. Nothing acts on that result, so the next tick tries again.

Removing the empty folders makes exists false, so the fallback kicks in. That is a manual workaround, not a fix.

Ask

1. Treat not_a_repository from a thread's worktree as permanent. After N results in a row (say 3), mark the worktree missing and stop PR polling for that thread until its worktree changes.
2. Or check for a git worktree (a .git file or folder) instead of plain path existence before choosing the worktree as cwd.
3. Optional: show "worktree missing" on the thread, so the user can archive it or recreate it.

Repro

1. Create a thread with a worktree and a branch, and leave it unsettled.
2. Delete everything inside the worktree folder, including .git, but keep the empty folder.
3. Watch the server log: not_a_repository from GitManager.branchPullRequest.remotes every poll, forever.