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/API.md

@@ -1006,6 +1006,9 @@ cannot collide on `index.lock`.
   the file resolves by removing it. `merge-commit` is `409 ECONFLICTS` while
   anything is unmerged, and it and `merge-abort` are `409 ENOMERGE` with no
   merge in progress. `merge-abort` needs `confirm: true` (`400 ECONFIRM`).
+- Any action on a host with no `git` binary (or none on the dashboard's PATH)
+  is `503 EGITMISSING`: a missing dependency, not a bad request; the message
+  carries the install command (like `ETMUXMISSING` for tmux).
 - `push` pushes `HEAD`, never a branch name from the request, so a workspace can
   only push the branch it is on.
 - A **path** in the body is never trusted as a path: `stage`, `unstage` and