fix(bootstrap): never stop the dashboard from inside its own systemd cgroup

A plugin self-update took the dashboard down and left it down. The chain
ran correctly up to the point where the bootstrap SIGTERMs the server it is
replacing — and that server was systemd-managed, with the bootstrap running
as its descendant, i.e. inside its cgroup. Two failures fired at once:

  * the server exits 0, and `Restart=on-failure` reads a clean exit as
    "meant to stop", so systemd did not bring it back;
  * `KillMode=control-group` reaped the bootstrap itself, mid-run, before it
    could repoint the unit at the new plugin root or write state.json.

Evidence from the incident: bootstrap.log ends at "stopped the server
started from the previous plugin version" with no systemd line after it,
.bootstrap.lock survived with a dead pid, state.json still named the
previous version, and the unit's ExecStart still pointed at the old cache
dir. The same truncation is visible on three earlier updates — those only
recovered because a later SessionStart bootstrapped again.

systemd-service.js now splits install into writeUserService (repoints the
unit, starts nothing) and restartUserService (--no-block). The bootstrap
skips the manual stop whenever a unit of ours owns the server, and defers
the restart past writeState and releaseLock, so being killed by its own
request costs nothing. The unit is Restart=always: a graceful SIGTERM from
anything must no longer read as "leave it stopped".

installUserService keeps its old semantics for `agenthub service install`,
which runs from a user's shell and is not in the cgroup.

CLAUDE.md

@@ -1,6 +1,6 @@
 # Claude Code Working Guide
 
-_Last synced: 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: v1.5.6 (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.
@@ -43,6 +43,7 @@ A **lane** is a durable unit of parallel agent work — one working directory, m
 See `docs/LANES.md` for full guide: stage reporting, custom templates, lane actions, and the five node states.
 
 ## 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.
 - **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.