perf: lighter, louder local install — vendored ReDoc, bootstrap failures shown in the session

- Plugin bootstrap: a failed npm install, a Node older than 22.5 or a failed UI
  build is recorded in runtime/last-failure.json; the next SessionStart prints
  it once as a systemMessage naming the log and /agenthub-doctor (a different
  failure is reported again; a successful bootstrap clears it). The foreground
  pass checks the Node version itself and spawns no worker on an old Node.
- ReDoc: the standalone bundle (redoc 2.5.4, MIT, licence files kept) is
  vendored under server/vendor/redoc/ and the `redoc` dependency dropped —
  a runtime install (npm ci --omit=dev --ignore-scripts) goes 126 MB -> 45 MB.
  vendor/ is excluded from the header audit and prettier.
- git missing from PATH: lane git actions answer 503 EGITMISSING with the
  install command instead of a bare ENOENT.
- package.json engines: node >=22.5.0 (node:sqlite), matching MIN_NODE.
- Docs: INSTALL.md is the one install/setup guide (SETUP.md merged in and now a
  pointer); no VS Code extension claim, one client-install hint, no
  windows-build-tools, better-sqlite3 notes scoped to checkouts, new
  "Plugin bootstrap never comes up" troubleshooting entry.

skills/file-headers/SKILL.md

@@ -23,7 +23,7 @@ that is missing the header, add it as part of the same change.
 | Included | Excluded |
 | -------- | -------- |
 | `*.js`, `*.ts`, `*.tsx`, `*.cjs`, `*.mjs` | anything under `node_modules/`, `dist/`, `build/`, `data/` |
-| `*.py`, `*.sh` | vendored/minified files (`*.min.js`) |
+| `*.py`, `*.sh` | vendored/minified files (`*.min.js`, anything under a `vendor/` directory) |
 | `*.css` | generated files (they carry their own AUTO-GENERATED banner) |
 | | snapshots (`__snapshots__/`), lockfiles, JSON/YAML/Markdown |
 | | anything **gitignored** — local runtime state that happens to live in the checkout (a workspace's own `.agenthub/profile/hooks/*.sh`, a scratch venv) is not this repo's source and can never be made compliant here |