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__/worktree.test.js

@@ -465,3 +465,23 @@ describe("listBranches", () => {
     assert.deepEqual(branches, []);
   });
 });
+
+describe("git missing from PATH", () => {
+  it("answers EGITMISSING with the fix, not a bare ENOENT, when the folder exists", async () => {
+    const saved = process.env.PATH;
+    process.env.PATH = path.join(ROOT, "no-such-bin");
+    try {
+      await assert.rejects(
+        () => wt.git(ROOT, ["status"]),
+        (err) => err.code === "EGITMISSING" && /install/i.test(err.message)
+      );
+      // A missing FOLDER is the other ENOENT, and stays one.
+      await assert.rejects(
+        () => wt.git(path.join(ROOT, "gone"), ["status"]),
+        (err) => err.code === "ENOENT"
+      );
+    } finally {
+      process.env.PATH = saved;
+    }
+  });
+});