ci: add mdbook docs build and deploy to GitHub Pages

Author: Claude Sonnet (coordinator)

.github/workflows/ci.yml

@@ -96,4 +96,32 @@ jobs:
           tag_name: ${{ env.VERSION }}
           files: dist/ledgerr-mcp-x86_64-unknown-linux-gnu.mcpb
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+  docs:
+    needs: test-and-build
+    if: github.event_name == 'push' && (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/heads/feature/'))
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Install mdbook
+        run: |
+          cargo install mdbook --locked
+          cargo install mdbook-mermaid --locked
+          cargo install mdbook-linkcheck --locked
+
+      - name: Build mdbook
+        run: mdbook build book
+
+      - name: Deploy to GitHub Pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./book/book