ci: use npm ci with caching, validate typedoc links, align Node versions (#533)

- Switch npm install -> npm ci in ci.yml (build, build-wsl) and docs.yml
  to fail on lockfile drift rather than silently tolerating it
- Add cache: npm to all setup-node steps that have a lockfile to hash
  (skipped test-git-install which has no checkout, and WSL which has no
  setup-node action)
- Add TypeDoc link validation step after build (Linux x64 only, matching
  existing single-OS gate pattern for MCPB). Verified passing on main.
- Align all workflows on Node 22 (active LTS) — previously CI used 20
  but publish workflows used 22, so releases were tested on a different
  Node than PRs
- Add engines.node >=20 to package.json so consumers on Node 18 get a
  warning

package.json

@@ -9,6 +9,9 @@
   "license": "MIT",
   "description": "MCP Apps SDK — Enable MCP servers to display interactive user interfaces in conversational clients.",
   "type": "module",
+  "engines": {
+    "node": ">=20"
+  },
   "main": "./dist/src/app.js",
   "exports": {
     ".": {