docs: add contributing guide and PR title lint

.github/workflows/lint-pr-title.yml

@@ -0,0 +1,31 @@
+name: lint-pr-title
+
+on:
+  pull_request:
+    types: [opened, edited, reopened, synchronize]
+
+permissions:
+  pull-requests: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            chore
+            docs
+            refactor
+            test
+            ci
+            perf
+            build
+          requireScope: false
+          subjectPattern: ^(?![A-Z]).+$
+          subjectPatternError: |
+            The PR title subject must start with a lowercase letter.

README.md

@@ -25,6 +25,23 @@ case-study/
     └── metrics-guide.md         # Impact metrics discovery, analytics MCP
 ```
 
+## Contributing
+
+Releases are automated by [release-please](https://github.com/googleapis/release-please), which derives the next version from [Conventional Commits](https://www.conventionalcommits.org/). The commit message (or PR title, if you squash-merge) determines the bump:
+
+| Prefix | Example | Effect |
+|---|---|---|
+| `fix:` | `fix: handle empty diff in design-engineer mode` | patch (1.4.1 → 1.4.2) |
+| `feat:` | `feat: add metrics-guide reference` | minor (1.4.1 → 1.5.0) |
+| `feat!:` or `BREAKING CHANGE:` in body | `feat!: rename SKILL.md frontmatter` | major (1.4.1 → 2.0.0) |
+| `chore:`, `docs:`, `refactor:`, `test:`, `ci:` | `docs: clarify install step` | no release |
+
+Use the imperative mood ("add", not "added"). Keep the subject under ~72 chars; put rationale in the body.
+
+### Enforcement
+
+PR titles are linted by `.github/workflows/lint-pr-title.yml` — non-conforming titles block the merge. For direct pushes to `main`, write a conforming commit message; non-`feat`/`fix` commits are simply ignored by release-please rather than rejected.
+
 ## Credits
 
 Built by [Jakub Salmik](https://jakubsalmik.com).