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/update-project-docs/references/doc-map.md

@@ -46,7 +46,7 @@ i18n architecture: **Supported languages** list, `supportedLngs`, the 15 namespa
 ## Tier 3 — situational
 
 - `.env.example` — every env var belongs here with a sane default + comment.
-- `INSTALL.md`, `SETUP.md` — install/run commands.
+- `INSTALL.md` — install/run commands, configuration, troubleshooting (`SETUP.md` is only a pointer to it).
 - `CLAUDE.md`, `AGENTS.md` — agent working guides; update when commands, file locations, or workflows change.
 - `docs/README.md` — docs index; add a link when a new `docs/*.md` is created.