feat(worktree): git worktree per task to isolate shared /repos (ORCH-2 / S-4)

- add src/git_worktree.py: ensure/remove/get_worktree_path
- config: worktrees_dir=/repos/_wt
- launcher: agent runs in per-branch worktree; task-file + commit/push in worktree; no shared checkout
- qg/checks: read artifacts + run make test from worktree (branch arg, backward-compatible)
- webhooks/plane: pass branch into QG dispatch; review fallback from worktree
- webhooks/gitea: keep read-only branch --contains in main clone (documented)
- tests: test_git_worktree.py (isolation) + update test_launcher write-task-file
- docs: ARCHITECTURE worktree section + BUGFIXES_2026-06-02_ORCH2

Preserves B-1/B-2/S-1/S-5 fixes (paths now point at worktree).

src/webhooks/gitea.py

@@ -146,7 +146,9 @@ async def handle_ci_status(payload: dict):
     if not branch:
         sha = payload.get("sha", "")
         repo_name = payload.get("repository", {}).get("name", settings.default_repo)
-        # Try to find task by checking git branch containing this SHA
+        # Try to find task by checking git branch containing this SHA.
+        # ORCH-2 / S-4: this is a READ-ONLY query of remote-tracking refs in the main
+        # clone (no checkout / no mutation), so it is safe to keep on /repos/<repo>.
         try:
             result = subprocess.run(
                 ["git", "-C", os.path.join(settings.repos_dir, repo_name),