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/__tests__/api.test.js

@@ -213,6 +213,13 @@ describe("OpenAPI / Swagger", () => {
     const res = await fetch("/api/redoc/redoc.standalone.js");
     assert.equal(res.status, 200);
     assert.match(res.headers["content-type"], /javascript/);
+    // The vendored file (server/vendor/redoc/), licence banner intact — the
+    // `redoc` npm package is no longer a dependency.
+    assert.match(
+      res.body,
+      /For license information please see redoc\.standalone\.js\.LICENSE\.txt/
+    );
+    assert.doesNotThrow(() => require("fs").statSync(require("../lib/redoc").redocBundlePath()));
   });
 });