ci: add PR size and title checks, configure commitizen

Add two new GitHub Actions workflows:
- pr-size.yml: auto-labels PRs by size and fails CI on PRs over 500 lines
- pr-title.yml: enforces conventional commit format on PR titles

Configure commitizen in pyproject.toml with strict schema pattern matching
for conventional commits.

.github/workflows/pr-title.yml

@@ -0,0 +1,38 @@
+name: PR Title Check
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+jobs:
+  pr-title:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            refactor
+            perf
+            test
+            docs
+            chore
+            ci
+            style
+            revert
+          requireScope: false
+          subjectPattern: ^[a-z].+[^.]$
+          subjectPatternError: >
+            The PR title "{title}" does not follow conventional commit format.
+
+            Expected: <type>(<scope>): <lowercase description without trailing period>
+
+            Examples:
+              feat(memory): add lancedb storage backend
+              fix(agents): resolve deadlock in concurrent execution
+              chore(deps): bump pydantic to 2.11.9
+
+            See RELEASE_PROCESS.md for the full commit message convention.