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/release/SKILL.md

@@ -0,0 +1,57 @@
+---
+name: release
+description: Create a new CalVer release with validation checks
+disable-model-invocation: true
+---
+
+# Release Workflow
+
+Follow these steps to create a new CalVer release:
+
+## 1. Pre-flight checks
+
+Run the full validation pipeline:
+
+```bash
+make all
+```
+
+If any step fails, fix the issues before proceeding.
+
+## 2. Check version references
+
+Verify all action references are properly pinned:
+
+```bash
+make check-version-refs
+make check-local-refs
+```
+
+## 3. Prepare the release
+
+Run release preparation (updates version references):
+
+```bash
+make release-prep
+```
+
+Review the changes with `git diff`.
+
+## 4. Confirm with user
+
+Ask the user to confirm:
+
+- The version number (defaults to `vYYYY.MM.DD` based on today's date)
+- That all changes look correct
+
+## 5. Create the release
+
+```bash
+make release VERSION=vYYYY.MM.DD
+```
+
+Replace `vYYYY.MM.DD` with the confirmed version.
+
+## 6. Verify
+
+Show the user the created tag and any output from the release process.