fix(plugin): build the UI with devDependencies so a self-update ships its own bundle

After the 1.5.2 -> 1.5.3 self-update the server ran 1.5.3 while the dashboard
still served the 1.5.2 UI. The client build had failed, and `runtime/client-dist`
still held the previous version's bundle, so nothing looked broken — the only
signal was the Settings page's "UI build v1.5.2 (rebuild client to match)".

Cause: the whole client build toolchain (vite, typescript,
@vitejs/plugin-react, tailwindcss, @types/*) is a devDependency, and npm reads
NODE_ENV=production as --omit=dev (`NODE_ENV=production npm config get omit` ->
`dev`). The dashboard server runs with NODE_ENV=production, and every bootstrap
descended from it inherits that: the plugin-update chain, and any SessionStart
fired by a `claude` running in a tmux pane the server started. So `npm ci`
dropped vite, `tsc -b` died on missing type declarations, and buildClient
returned "failed" with the stale bundle left in place.

`--include=dev` on both install paths — a CLI flag outranks the env, which is
the point. Verified by running buildClient with NODE_ENV=production against the
installed plugin root: previously "failed", now "built", and the served bundle
carries 1.5.3.

Also: plugin-update-helper writes its verdict BEFORE the bootstrap handover as
well as after. That step retires the process tree the helper hangs off, and the
observed run reached the new server with no verdict file at all — the dashboard
reported `last_plugin_update: null` for an update that had actually succeeded.

docs/PLUGINS.md

@@ -134,7 +134,7 @@ waits on an install:
 4. **Install** — `npm install --omit=dev --ignore-scripts` into the runtime dir (never into the plugin cache, which is garbage-collected and replaced on every update). `--ignore-scripts` keeps the root `postinstall` from pulling the whole Vite client toolchain.
 5. **Legacy hook cleanup** — see above.
 6. **CLI** — write the `~/.local/bin/agenthub` launcher, never clobbering a `agenthub` the bootstrap did not write.
-7. **UI build** — `client/` is copied into the runtime dir and built there (`npm install && npm run build`), landing in `runtime/client-dist` — the same directory the server serves from. Skipped when a bundle for the current version already exists; a failure here does not fail the bootstrap (API and MCP still work, and `/agenthub-open` can retry) but does mean `/run` 404s until it's fixed.
+7. **UI build** — `client/` is copied into the runtime dir and built there (`npm ci --include=dev`, falling back to `npm install --include=dev`, then `npm run build`), landing in `runtime/client-dist` — the same directory the server serves from. `--include=dev` is required, not tidy: the entire build toolchain (`vite`, `typescript`, `@vitejs/plugin-react`, `tailwindcss`, `@types/*`) is a devDependency, and npm reads an inherited `NODE_ENV=production` as `--omit=dev`. Every bootstrap descended from the running server carries that env — the self-update chain, and any `SessionStart` fired by a `claude` inside a tmux pane the server started — so without the flag the install drops `vite` and the build dies on missing types. Skipped when a bundle for the current version already exists; a failure here does not fail the bootstrap (API and MCP still work, and `/agenthub-open` can retry) but does mean `/run` 404s until it's fixed — and when a bundle from the PREVIOUS version is still on disk the failure is quieter still: the dashboard keeps serving the old UI against the new API, visible only as the Settings page's `UI build vX (rebuild client to match)` line.
 8. **Server** — on Linux with `systemd --user` available, install/refresh a
    `agenthub.service` unit (see [Auto-start on boot](#auto-start-on-boot)) instead
    of a bare spawn, so the server is supervised and comes back after a reboot