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/vendor/redoc/LICENSE

@@ -0,0 +1,22 @@
+The MIT License (MIT)
+
+Copyright (c) 2015-present, Rebilly, Inc. 
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+

server/vendor/redoc/redoc.standalone.js.LICENSE.txt

@@ -0,0 +1,102 @@
+/*!
+	Copyright (c) 2018 Jed Watson.
+	Licensed under the MIT License (MIT), see
+	http://jedwatson.github.io/classnames
+*/
+
+/*!
+ * ReDoc - OpenAPI/Swagger-generated API Reference Documentation
+ * -------------------------------------------------------------
+ *   Version: "2.5.4"
+ *   Repo: https://github.com/Redocly/redoc
+ */
+
+/*!
+ * Stickyfill -- `position: sticky` polyfill
+ * v. 1.1.1 | https://github.com/wilddeer/stickyfill
+ * Copyright Oleg Korsunsky | http://wd.dizaina.net/
+ *
+ * MIT License
+ */
+
+/*!
+ * perfect-scrollbar v1.5.3
+ * Copyright 2021 Hyunje Jun, MDBootstrap and Contributors
+ * Licensed under MIT
+ */
+
+/*! @license DOMPurify 3.4.13 | (c) Cure53 and other contributors | Released under the Apache license 2.0 and Mozilla Public License 2.0 | github.com/cure53/DOMPurify/blob/3.4.13/LICENSE */
+
+/*!***************************************************
+* mark.js v8.11.1
+* https://markjs.io/
+* Copyright (c) 2014–2018, Julian Kühnel
+* Released under the MIT license https://git.io/vwTVl
+*****************************************************/
+
+/**
+ * @license React
+ * react-dom.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * react.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * scheduler.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * use-sync-external-store-shim.production.js
+ *
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * Prism: Lightweight, robust, elegant syntax highlighting
+ *
+ * @license MIT <https://opensource.org/licenses/MIT>
+ * @author Lea Verou <https://lea.verou.me>
+ * @namespace
+ * @public
+ */
+
+/** @license React v16.13.1
+ * react-is.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/** @license React v17.0.2
+ * react-is.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */