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.

server/routes/lanes.js

@@ -1937,7 +1937,10 @@ router.post("/:id/git/:action", sameOriginGuard, async (req, res) => {
             "EMEMBERGONE",
           ].includes(err.code)
         ? 409
-        : 500;
+        : // Not the request's fault and not a server bug: the host lacks git.
+          err.code === "EGITMISSING"
+          ? 503
+          : 500;
     // git's own stderr is the useful half of a failed push or pull (a
     // rejection, a missing credential, a diverged branch), so it joins the
     // message rather than riding a side field the client would have to know