chore(claude): add hooks, skills, and agents for Claude Code (#496)

* 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.

* fix(claude): add tool availability guards and fix skill docs

Add jq availability checks to hook scripts (block-rules-yml.sh,
post-edit-write.sh) and wrap actionlint call in command -v guard,
consistent with project rules #2 and #10. Fix validate skill to
reflect actual make all pipeline order and note that make test
runs separately.

* fix(claude): correct skill docs per PR review feedback

Fix validate skill description to say "precommit" instead of "test",
and fix check-pins SHA guidance to use origin/main instead of HEAD.

* feat(tools): add SHA-pinning enforcement to check-version-refs

The check-version-refs script previously only displayed existing
SHA-pinned refs but silently skipped non-SHA references. Add a
validation pass that detects and reports any ivuorinen/actions/*
references not using a 40-char hex SHA, exiting 1 on violations.

* fix(tools): fix temp file leak in check-version-refs.sh

Write find output directly to $violations_file instead of
$violations_file.all so the EXIT trap covers cleanup on all
exit paths, not just the happy path.

.claude/skills/validate/SKILL.md

@@ -0,0 +1,51 @@
+---
+name: validate
+description: Run full validation pipeline (docs, format, lint, precommit)
+disable-model-invocation: true
+---
+
+# Full Validation Pipeline
+
+Run the complete validation pipeline:
+
+```bash
+make all
+```
+
+This runs in order: `install-tools` -> `update-validators` -> `docs` -> `update-catalog` -> `format` -> `lint` -> `precommit`
+
+**Note:** `make test` must be run separately.
+
+## 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.