chore: migrate legacy pre-commit hook to .pre-commit-config.yaml

Replaces shell script (.git/hooks/pre-commit.legacy) with ecosystem-managed pre-commit framework:
- gitleaks/gitleaks: secret scanning
- jumanjihouse/pre-commit-hooks: shellcheck wrapper
- pre-commit/pre-commit-hooks: JSON/YAML validation, end-of-file-fixer, trailing-whitespace
- local hooks: SKILL.md frontmatter validation

Uses pinned versions for reproducibility across environments. Includes auto-fixes from hook runs (formatting, trailing whitespace, JSON beautification).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -0,0 +1,40 @@
+repos:
+  - repo: https://github.com/gitleaks/gitleaks
+    rev: v8.21.2
+    hooks:
+      - id: gitleaks
+
+  - repo: https://github.com/jumanjihouse/pre-commit-hooks
+    rev: 3.0.0
+    hooks:
+      - id: shellcheck
+        args: [--severity=warning]
+
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.5.0
+    hooks:
+      - id: end-of-file-fixer
+      - id: check-json
+      - id: pretty-format-json
+      - id: check-yaml
+      - id: trailing-whitespace
+
+  - repo: local
+    hooks:
+      - id: skill-frontmatter
+        name: SKILL.md frontmatter validation
+        description: Ensure SKILL.md files have required frontmatter fields
+        entry: bash
+        language: system
+        files: 'SKILL\.md$'
+        args:
+          - -c
+          - |
+            for f in "$@"; do
+              if [[ -f "$f" ]]; then
+                if ! grep -q "^name:" "$f" || ! grep -q "^description:" "$f"; then
+                  echo "ERROR: $f is missing required frontmatter fields (name: and description:)"
+                  exit 1
+                fi
+              fi
+            done