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/lib/worktree.js

@@ -115,6 +115,19 @@ async function git(
     });
     return { stdout: result.stdout, stderr: result.stderr };
   } catch (err) {
+    // ENOENT from execFile is ambiguous: a missing binary OR a missing cwd.
+    // With the folder present it is git itself that is absent — say so, with
+    // the fix, rather than `git status failed` (like ETMUXMISSING for tmux).
+    if (err.code === "ENOENT" && cmd === "git" && fs.existsSync(cwd)) {
+      const error = new Error(
+        "git is not installed (or not on the dashboard's PATH), and every git action needs it. " +
+          "Install it (sudo apt install git / brew install git / https://git-scm.com/download/win), " +
+          "then restart the dashboard."
+      );
+      error.code = "EGITMISSING";
+      error.git = { args, code: "EGITMISSING", stderr: "", stdout: "" };
+      throw error;
+    }
     const error = new Error(`git ${args[0]} failed`);
     error.code = err.code;
     error.git = { args, code: err.code, stderr: err.stderr, stdout: err.stdout };