Add basic-host and basic-server-react examples

Introduce new example implementations to replace simple-host and
simple-server:

- basic-host: Reference host implementation with double-iframe sandbox
  pattern for secure UI isolation (outer iframe validates and relays
  messages, inner iframe renders untrusted tool UI via srcdoc)

- basic-server-react: MCP server example demonstrating tool registration
  with linked UI resources and React UI using the useApp() hook

Build infrastructure updates:
- Add build:all script and simplify CI/pre-commit to use it
- Add examples:dev for watch-mode development workflow

Also change PostMessageTransport logging from info to debug level.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -25,18 +25,6 @@ jobs:
 
       - run: npm install
 
-      - run: npm run build
-
-      - name: Build simple-host example
-        working-directory: examples/simple-host
-        run: |
-          npm install
-          npm run build
-
-      - name: Build simple-server example
-        working-directory: examples/simple-server
-        run: |
-          npm install
-          npm run build
+      - run: npm run build:all
 
       - run: npm run prettier