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/settings.json

@@ -0,0 +1,26 @@
+{
+  "hooks": {
+    "PreToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/block-rules-yml.sh"
+          }
+        ]
+      }
+    ],
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/post-edit-write.sh"
+          }
+        ]
+      }
+    ]
+  }
+}