Add lint skill and update existing skills to reference it

Co-authored-by: afranken <763000+afranken@users.noreply.github.com>

Author: Copilot

.claude/skills/implement/SKILL.md

@@ -13,8 +13,9 @@ A complete feature implementation requires these skills — run them in sequence
 
 1. **`refactor` skill** — if nearby existing code, tests, or configuration would benefit from cleanup *before* adding new code, do it first rather than working around it
 2. **`implement` skill** (this skill) — write the production code
-3. **`test` skill** — add/update unit and integration tests (`*Test.kt`, `*IT.kt`)
-4. **`document` skill** — update `CHANGELOG.md`, `README.md`, and `AGENTS.md` where applicable
+3. **`lint` skill** — fix formatting and verify style gates pass
+4. **`test` skill** — add/update unit and integration tests (`*Test.kt`, `*IT.kt`)
+5. **`document` skill** — update `CHANGELOG.md`, `README.md`, and `AGENTS.md` where applicable
 
 > **Prefer refactoring over workarounds**: if you find yourself adding complexity to work around existing code, stop and invoke the `refactor` skill to clean it up first.
 
@@ -32,12 +33,12 @@ Follow **DTO → Store → Service → Controller** (see AGENTS.md Architecture)
 - [ ] Read root + module `AGENTS.md` (required before any other step)
 - [ ] Identify the S3 API operation ([AWS docs](https://docs.aws.amazon.com/AmazonS3/latest/API/Welcome.html))
 - [ ] Review existing similar implementations
-- [ ] Run `make format` then `make install`
+- [ ] Invoke the **`lint` skill** to fix formatting and verify style gates pass
 - [ ] Invoke the **`test` skill** to add/update unit and integration tests
 - [ ] Invoke the **`document` skill** to update `CHANGELOG.md`, `README.md`, and `AGENTS.md`
 
 ## Troubleshooting
 
-- **Build fails**: Check Java 25, run `make format`
+- **Build fails**: Check Java 25, invoke the **`lint` skill**
 - **Tests fail**: Ensure XML matches AWS API exactly — run integration tests
 - **Docker fails**: Try `make skip-docker` to isolate

.claude/skills/lint/SKILL.md

@@ -0,0 +1,53 @@
+---
+name: lint
+description: Fix code style issues and ensure linting passes. Use when asked to fix lint errors, formatting issues, or when ktlint or Checkstyle violations are reported.
+---
+
+# Lint Skill — S3Mock
+
+Read `AGENTS.md` (root + relevant module) before making changes.
+
+## Linters
+
+S3Mock uses two linting tools that run as required CI gates:
+
+| Tool | Target | Config | Auto-fix? |
+|------|--------|--------|-----------|
+| **ktlint** | Kotlin source files | `.editorconfig` | Yes — `make format` |
+| **Checkstyle** | Java source + XML files | `etc/checkstyle.xml` | No — fix manually |
+
+Both run automatically as part of the full build (`make install`).
+
+## Workflow
+
+1. **Run `make format`** — auto-formats all Kotlin files with ktlint. Fixes the vast majority of Kotlin style issues.
+2. **Run `./mvnw checkstyle:check`** — reports Checkstyle violations for Java and XML files. Fix violations manually.
+3. **Re-run `make install`** — confirm all linting gates pass before submitting.
+
+## ktlint (Kotlin)
+
+`make format` auto-fixes most issues. Common violations:
+- Wrong indentation (4 spaces for Kotlin)
+- Unused or wildcard imports
+- Missing trailing newline
+- Line length (max 120 characters — see `.editorconfig`)
+
+To check without modifying files: `./mvnw ktlint:check`
+
+## Checkstyle (Java / XML)
+
+Violations must be fixed manually. Common violations (config in `etc/checkstyle.xml`):
+- Wrong indentation (2 spaces for Java)
+- Line length (max 120 characters)
+- Import ordering
+- Missing or malformed Javadoc
+
+See **[docs/JAVA.md](../../../docs/JAVA.md)** for Java style conventions.
+
+## Checklist
+
+- [ ] Read root + module `AGENTS.md` for the files being changed
+- [ ] Run `make format` to auto-fix Kotlin style
+- [ ] Run `./mvnw checkstyle:check` to check Java/XML style
+- [ ] Fix any remaining violations manually (see `docs/JAVA.md` for Java conventions, `docs/KOTLIN.md` for Kotlin)
+- [ ] Re-run `make install` to confirm all CI gates pass

.claude/skills/refactor/SKILL.md

@@ -34,7 +34,7 @@ Document what, why, and gotchas. Link to AWS API docs where relevant. See **[doc
 ## Checklist
 
 - [ ] Verify no behavior changes — run tests
-- [ ] Run `make format`
+- [ ] Invoke the **`lint` skill** to fix formatting and verify style gates pass
 - [ ] Ensure comments explain *why*, not *what*
 - [ ] Add KDoc for all public APIs
 - [ ] Use self-documenting names

.claude/skills/review/SKILL.md

@@ -57,6 +57,6 @@ For each finding, reference the specific AGENTS.md rule or AWS API doc where app
 
 - [ ] Read root + relevant module `AGENTS.md`
 - [ ] Check all categories above in priority order
-- [ ] Verify CI gates will pass (ktlint, checkstyle, tests, Docker build)
+- [ ] Verify CI gates will pass (ktlint, checkstyle, tests, Docker build) — suggest invoking the **`lint` skill** if style issues are found
 - [ ] Confirm `CHANGELOG.md` is updated if needed
 - [ ] Provide actionable feedback with specific file/line references

.claude/skills/test/SKILL.md

@@ -24,4 +24,4 @@ Read **[docs/TESTING.md](../../../docs/TESTING.md)**, **[docs/KOTLIN.md](../../.
 - [ ] Cover both success and failure cases
 - [ ] Keep tests independent (no shared state, UUID bucket names)
 - [ ] Use specific assertions
-- [ ] Run `make format`
+- [ ] Invoke the **`lint` skill** to fix formatting and verify style gates pass

AGENTS.md

@@ -104,9 +104,11 @@ make install              # Full build
 make skip-docker          # Skip Docker
 make test                 # Unit tests only
 make integration-tests    # Run integration tests
-make format               # Format Kotlin code
+make format               # Format Kotlin code (ktlint)
 ```
 
+Use the **`lint` skill** to fix formatting and verify style gates (ktlint + Checkstyle) pass.
+
 ## CI/CD Pipeline
 
 All PRs and pushes are validated by the `maven-ci-and-prb.yml` GitHub Actions workflow.