Replace {@includeCode} with synced code fences

Migrate JSDoc and markdown documentation from `{@includeCode ./file#region}`
tags to inline code fences with `source="./file#region"` attributes.
Code examples are now visible directly in source files while remaining
type-checked via their `.examples.ts` source files.

Add `scripts/sync-snippets.ts` to extract code from `//#region` blocks
and sync it into labeled fences. Run with `npm run sync:snippets`.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -56,6 +56,12 @@ jobs:
           npm run generate:schemas
           git diff --exit-code src/generated/ || (echo "Generated schemas are out of date. Run 'npm run generate:schemas' and commit." && exit 1)
 
+      - name: Verify synced snippets are up-to-date
+        shell: bash
+        run: |
+          npm run sync:snippets
+          git diff --exit-code src/ docs/ || (echo "Synced snippets are out of date. Run 'npm run sync:snippets' and commit." && exit 1)
+
       - run: npm test
 
       - run: npm run prettier