feat(hub): add markdown_to_pdf tool and default web tools

Teachers need ad-hoc Markdown → PDF. Ship an MCP tool powered by
md-to-pdf (Marked + headless Chrome) so remote images/CSS work, with
workspace-scoped basedir, front-matter stripped so untrusted markdown
cannot override dest/basedir/launch options, and MathJax for $/$ math.

Also include WebFetch and WebSearch in the unrestricted role tool
surface by default. Deploy skips Puppeteer's browser download and
expects a host Chrome/Chromium (PUPPETEER_EXECUTABLE_PATH / CHROME_PATH).

hub/deploy/deploy_platform.sh

@@ -63,7 +63,7 @@ if [ "$release_ready" = false ]; then
   # 2. Install deps (hub + admin-web), audit hub prod, build tsc + SPA, mark complete.
   # `npm run build` → tsc then admin:build → admin-web/build for registerStaticSpa.
   ssh "${SSH_OPTS[@]}" "$DEPLOY_USER@$HOST" \
-    "cd '$HUB_DIR' && npm ci && npm ci --prefix admin-web && npm run audit:production && npm run build && touch '$RELEASE_DIR/.complete'"
+    "cd '$HUB_DIR' && PUPPETEER_SKIP_DOWNLOAD=1 npm ci && npm ci --prefix admin-web && npm run audit:production && npm run build && touch '$RELEASE_DIR/.complete'"
 fi
 
 # 3. Ensure the service is installed (idempotent), then restart.