chore(claude): add hooks, skills, and agents for Claude Code

Add auto-formatting hooks (ruff, shfmt, prettier, actionlint),
rules.yml edit blocker, 5 skills (/release, /test-action,
/new-action, /validate, /check-pins), and 2 subagents
(action-validator, test-coverage-reviewer). Update CLAUDE.md
with hook documentation.

.claude/skills/validate/SKILL.md

@@ -0,0 +1,49 @@
+---
+name: validate
+description: Run full validation pipeline (docs, format, lint, test)
+disable-model-invocation: true
+---
+
+# Full Validation Pipeline
+
+Run the complete validation pipeline:
+
+```bash
+make all
+```
+
+This runs in order: `docs` -> `format` -> `lint` -> `test`
+
+## If validation fails
+
+### Formatting issues
+
+```bash
+make format
+```
+
+Then re-run `make all`.
+
+### Linting issues
+
+- **actionlint**: Check action.yml syntax, step IDs, expression usage
+- **shellcheck**: POSIX compliance, quoting, variable usage
+- **ruff**: Python style and errors
+- **markdownlint**: Markdown formatting
+- **prettier**: YAML/JSON/MD formatting
+
+### Test failures
+
+```bash
+make test
+```
+
+Read the failing test output and fix the underlying action or test.
+
+### Documentation drift
+
+```bash
+make docs
+```
+
+Regenerates READMEs from action.yml files.