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.

docs/OVERVIEW.md

@@ -115,7 +115,7 @@ Internal build — all rights reserved.
 
 | Need | Version | Required for |
 |---|---|---|
-| **Node.js** | >= 20 (>= 22.5 for a plugin install) | Everything. Both suites are verified green on **24** and **26** |
+| **Node.js** | >= 22.5 (`node:sqlite`; Node 20 is end-of-life) | Everything. Both suites are verified green on **24** and **26** |
 | **npm** | 9+ | Ships with Node |
 | **git** | any | Cloning, and every workspace operation (worktrees, branch/status reads) |
 | **tmux** | 3.x | The browser run console, `agenthub workspace shell`, typing `/reload-plugins` into a live pane |