fix(bootstrap): stop a retired plugin build from downgrading the dashboard

`claude plugin update` unpacks the new version into a new cache directory and
says "Restart to apply changes". A Claude Code process that was already running
does not: it keeps loading the OLD directory, hooks and all.

Every session such a process opened then ran the OLD plugin-bootstrap, which saw
a state.json naming a different root, concluded the server had to be replaced,
repointed agenthub.service at itself and restarted the dashboard onto the
version the user had just upgraded away from. The dashboard reported the old
version and offered the update again — once per session, forever, while
installed_plugins.json and the cache both said the new version was installed.
A /agenthub-update from that session "succeeded" and changed nothing.

- `supersededBy()` reads Claude Code's installed_plugins.json. When a SIBLING
  cache directory is the recorded install, the bootstrap logs one line and
  returns "superseded" — no deps, no unit, no server. `--force` does not
  override it: the self-update chain forces from the NEW directory, which is
  never superseded, so the only --force refused is a deliberate downgrade.
  It cannot hand over either — the process it would replace is the one loading
  it — so the log line says to restart Claude Code.
- Matched by sibling directory, not by the <plugin>@<marketplace> key: correct
  without parsing a cache path or trusting a name. A phantom entry (installPath
  that no longer exists) is ignored rather than wedging the bootstrap shut.

Also adds `pruneRetiredPluginRoots()`, since the same updates leave a full copy
of the repo — node_modules and all — per release; twelve of them measured 1.7 GB
on one machine. It runs after writeState, so a bad prune cannot cost the record
of a good bootstrap, and it protects four independent readings of "in use":
this build, every installPath Claude Code records, the directory
agenthub.service names (`unitRoot`, foreign unit included), and any directory
still named in the user's settings.json hooks — that last one is the un-restarted
process above, seen from the other side. Plus the 2 newest survivors as a
rollback cushion, and nothing at all outside ~/.claude/plugins/cache/.

CLAUDE.md

@@ -1,6 +1,6 @@
 # Claude Code Working Guide
 
-_Last synced: v1.6.0 (2026-09-14) + AI Router profiles (`server/lib/ai-routers.js`, `server/lib/router-apply.js`, the Settings tab `/settings?tool=ai-router`, and `lanes.router_profile`) + v1.5.6 (2026-09-11) + the systemd handover rule below (`scripts/systemd-service.js`'s `writeUserService`/`restartUserService` split and `plugin-bootstrap.js`'s deferred `pendingSystemdRestart`) + eb63569 (2026-09-10) + release-based update detection (`server/lib/update-check.js` reads the forge releases API — a checkout via its `origin` remote, a plugin-cache install via the marketplace manifest on disk, since an archive-sourced cache dir has no `.git`; a `plugin_managed` install self-updates via the detached `plugin-update` chain) + the install-wide **Update all** batch and session reload (`server/lib/cc-update-all.js`, `server/lib/cc-reload.js`)._ A change that makes any statement in this file, `AGENTS.md`, or a README wrong — a moved path, a new or changed extension point, a retired rule — updates that file in the same change-set and bumps this marker.
+_Last synced: v1.6.0 (2026-09-14) + AI Router profiles (`server/lib/ai-routers.js`, `server/lib/router-apply.js`, the Settings tab `/settings?tool=ai-router`, and `lanes.router_profile`) + the bootstrap's superseded-build guard and plugin-cache pruner (`supersededBy`/`pruneRetiredPluginRoots` in `scripts/plugin-bootstrap.js`, `unitRoot` in `scripts/systemd-service.js`) + v1.5.6 (2026-09-11) + the systemd handover rule below (`scripts/systemd-service.js`'s `writeUserService`/`restartUserService` split and `plugin-bootstrap.js`'s deferred `pendingSystemdRestart`) + eb63569 (2026-09-10) + release-based update detection (`server/lib/update-check.js` reads the forge releases API — a checkout via its `origin` remote, a plugin-cache install via the marketplace manifest on disk, since an archive-sourced cache dir has no `.git`; a `plugin_managed` install self-updates via the detached `plugin-update` chain) + the install-wide **Update all** batch and session reload (`server/lib/cc-update-all.js`, `server/lib/cc-reload.js`)._ A change that makes any statement in this file, `AGENTS.md`, or a README wrong — a moved path, a new or changed extension point, a retired rule — updates that file in the same change-set and bumps this marker.
 
 ## Project mission
 - Maintain a reliable local-first dashboard for Claude Code session monitoring.
@@ -45,6 +45,8 @@ See `docs/LANES.md` for full guide: stage reporting, custom templates, lane acti
 
 ## Non-negotiable engineering rules
 - **When a `agenthub.service` unit of ours owns the server, nothing else may stop it.** The bootstrap that performs a self-update is spawned by the running server, so it lives in that server's systemd cgroup: SIGTERM-ing the server from there exits 0 (which `Restart=on-failure` read as "meant to stop") AND trips `KillMode=control-group`, reaping the bootstrap before it can repoint the unit or write `state.json` — a dashboard dead after every update until some later session bootstrapped again. So `scripts/systemd-service.js` splits the API: `writeUserService` repoints the unit and starts NOTHING, `restartUserService` uses `--no-block`, and `plugin-bootstrap.js` defers that restart past `writeState` and `releaseLock` (`pendingSystemdRestart`). The unit is `Restart=always`. Do not add a path that stops the dashboard from inside its own cgroup, and do not move the restart earlier. `agenthub service install` may keep using `installUserService` — it runs from a user's shell, outside the cgroup. `systemdOwned` additionally requires the unit to be ACTIVE (an installed-but-stopped unit owns nothing; treating it as owner skips the stop and then binds an occupied port). A unit naming a RETIRED plugin root is invisible until a reboot, so `fastPathOk` rejects it via `unitIsStale` (pure fs — that path may never spawn a process) and the bootstrap rewrites it; repairing the unit is NOT restarting the server (`pendingSystemdRestart = systemdOwned && needsServer`), and the detached-spawn fallback is guarded by `needsServer` so a stale unit alone never starts a second server onto a bound port.
+- **A bootstrap that is no longer the installed build does NOTHING, and `--force` does not override that.** `supersededBy` (`scripts/plugin-bootstrap.js`) reads Claude Code's `installed_plugins.json`: if a SIBLING cache directory is the recorded install, this build logs one line and returns `superseded` — no deps, no unit, no server. A Claude Code process that was not restarted after an update keeps loading the OLD cache dir, and without this its every session repointed `agenthub.service` at that old dir and restarted the dashboard onto the version the user just upgraded away from, once per session, forever. It may not hand over to the new build either — the process it would replace is the one loading it. Do not add a path that lets a retired root write the unit, stop the server, or "just" refresh deps.
+- **Deleting retired cache copies is the ONE thing here that touches the plugin cache, and four protections gate it.** `pruneRetiredPluginRoots` never deletes: this build; any `installPath` in `installed_plugins.json`; the dir `agenthub.service` names (`unitRoot`, foreign unit included); any dir named in the user's `settings.json` hooks (a un-restarted Claude Code is still executing from there — deleting it breaks that session's hooks). Plus the 2 newest survivors as a rollback cushion, and nothing at all outside `~/.claude/plugins/cache/`. It runs AFTER `writeState`, so a bad prune cannot also cost the record of a good bootstrap. Adding a fifth thing that pins a cache dir means adding a fifth protection here.
 - **A session reload is typed into a live pane, never spawned.** `/reload-plugins` and `/reload-skills` are scoped BY CLAUDE CODE to the session that runs them, so nothing can reload a session from the outside. `server/lib/cc-reload.js` therefore types them into the dashboard's existing tmux lane panes and spawns NO `claude` process — a one-shot `claude -p "/reload-plugins"` reloads only its own short-lived session, exits, and leaves a junk session in the dashboard's own history. Do not add a path that starts a session in order to reload one.
 - Preserve existing behavior unless explicitly asked to change it.
 - Prefer minimal, reversible diffs.