docs(relay): add worktree-autodetect relay packet

Assessment of auto-detecting worktrees created outside PI WEB. Worktree
discovery is already derived per request with no cache and no registry, so
the gap is that the browser never re-lists. Recommends the reduced scope:
filter prunable worktrees, add a non-disruptive topology refresh, and call
it from the existing browser-resume path. No watchers, timers, processes, or
push channels.

Packet is repo-only and outside the published files allowlist, so no
changeset is needed.

.pi-web/relays/worktree-autodetect/status.md

@@ -0,0 +1,90 @@
+# Status — relay "worktree-autodetect"
+
+## ✅ APPROVED — relay is live
+
+The human approved the reduced scope in leg 0 and answered every open question. There are
+no outstanding decisions. Run leg 1.
+
+## Current position
+
+Leg 0 (assessment/design) is complete. No production code has been written.
+
+The recommendation is **reduced scope**: detection piggybacked on browser resume, plus a
+fix for the inverse (removed-worktree) case. No watchers, no timers, no new processes,
+no new push channel. Rationale is in `log.md` leg 0; the implementation breakdown is in
+`plan.md`.
+
+## Leg tracking
+
+- **Last completed leg:** 0 (assessment and packet creation)
+- **Next leg to run:** 1
+
+## Next task — leg 1
+
+Stop reporting worktrees whose checkout directory has been removed outside PI WEB.
+
+See `plan.md` → "Leg 1". Summary: teach the `git worktree list --porcelain` parser about
+the `prunable` and `locked` keys, exclude prunable linked worktrees from the workspace
+list, never filter the main worktree, never mutate the repo (no `git worktree prune`).
+
+Files: `src/server/workspaces/gitWorktreeDiscovery.ts`, `src/server/workspaces/workspaceService.ts`,
+new `src/server/workspaces/gitWorktreeDiscovery.test.ts`.
+
+## Relevant context for the next runner
+
+Facts established in leg 0 — trust these, they were verified against the running code and
+real git; do not re-derive them:
+
+- **Worktrees are already derived, never registered.** `WorkspaceService.list()` shells out
+  to `git worktree list --porcelain` on **every** `GET /projects/:projectId/workspaces`
+  request. There is no server-side cache and no invalidation to design. A worktree created
+  outside PI WEB is *already* discovered — the gap is purely that the browser never re-asks.
+- **`projects.json` holds projects only, not workspaces.** So "auto-adoption" is a
+  non-problem: nothing needs to be written to a registry, and there is no adopt-vs-visible
+  distinction to design. This collapsed most of the feature's apparent complexity.
+- **Cost of discovery is ~2ms** (measured: 20 sequential `git worktree list --porcelain`
+  runs in 42ms on this repo).
+- **`prunable` is real and load-bearing.** After `rm -rf`ing a worktree directory without
+  `git worktree remove`, `git worktree list --porcelain` still lists it, with an added
+  `prunable gitdir file points to non-existent location` line. PI WEB currently shows this
+  as a normal selectable workspace. `locked` appears as a bare valueless line.
+- **The resume path already exists and is already debounced.**
+  `src/client/src/appShell/browserResumeController.ts` listens to window `focus` and
+  document `visibilitychange`, batches per animation frame, and collapses concurrent
+  requests via `TrailingRefreshCoordinator`. It calls
+  `PiWebApp.refreshAfterBrowserResume()` (~line 432), which already refreshes the selected
+  session, machine activities, and workspace-deletion runs. Workspace topology is the one
+  thing missing from that list.
+- **Remote machines come for free.** `workspacesApi.workspaces(projectId, machineId)`
+  routes through `machinePrefix`, and `GET /projects/:projectId/workspaces` is already in
+  `FEDERATED_HTTP_ROUTES` in `src/shared/federatedRoutes.ts`. No transport work needed.
+- **The danger is `selectWorkspace`.** It calls `clearActiveSession()` and
+  `resetWorkspaceScopedState()`. A refresh must apply the new list via
+  `applyProjectWorkspaces` **only**, and must not route through `selectWorkspace` when the
+  selection is still valid. `ProjectActivityOwnershipCoordinator` is the existing precedent
+  for background topology hydration that does not disturb selection — read it if leg 2
+  needs a model.
+- **No session daemon involvement.** Nothing in this design touches `src/server/sessiond.ts`,
+  session runtime ownership, or the daemon protocol. **No manual sessiond restart needed.**
+
+## Progress documentation expected of each runner
+
+- Commit the slice (Conventional Commit message).
+- Update this file: current position, leg tracking, next task, blockers.
+- Append to `log.md`: what, why, artifacts, exact checks run and results.
+- Add the `.changeset/*.md` fragment no later than leg 3.
+
+## Decisions settled by the human (do not re-open)
+
+1. **Latency: resume-scoped is acceptable.** Detection on tab refocus/visibility is the
+   agreed behavior. Do not add a timer or a watcher to shorten it.
+2. **Removed worktrees: hide them.** Filtering `prunable` worktrees out of the workspace
+   list is wanted and approved.
+3. **Sibling overlap: assume the other session does nothing.** This relay owns the
+   workspace-topology refresh seam outright. Build it here, do not design for sharing,
+   and do not read `/srv/dev/pi-web-worktrees/worktree-create-ui`. If that branch later
+   merges something overlapping, resolving it is that branch's problem, not this one's.
+
+## Blockers
+
+None. Leg 1 is clear to run.