chore(mcp): add CI workflow and document cross-cutting changes

- .github/workflows/mcp-ci.yml: runs on pushes to main and PRs
  touching packages/mcp/, packages/core/, or bun.lock. Installs with
  Bun 1.3.0, builds core then mcp, runs bun test, and runs biome.
- packages/mcp/CROSS_CUTTING.md: documents the two additive changes
  outside the package boundary (core exports map, this CI workflow)
  plus observations about SiteNode.children inconsistency.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/mcp-ci.yml

@@ -0,0 +1,41 @@
+name: mcp-ci
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'packages/mcp/**'
+      - 'packages/core/**'
+      - 'bun.lock'
+  pull_request:
+    paths:
+      - 'packages/mcp/**'
+      - 'packages/core/**'
+      - 'bun.lock'
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: 1.3.0
+
+      - name: Install
+        run: bun install --frozen-lockfile
+
+      - name: Build core
+        run: bun run --cwd packages/core build
+
+      - name: Build mcp
+        run: bun run --cwd packages/mcp build
+
+      - name: Test mcp
+        run: bun test --cwd packages/mcp
+
+      - name: Biome check
+        run: bunx biome check packages/mcp