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>

docs/prd/chunk-3-skills-library.md

@@ -1,7 +1,7 @@
 # PRD: Chunk 3 — Skills Library Rebuild
 
-**Status:** In progress — 0015 ✅, 0016 ✅  
-**Produced by:** grill-with-docs session, 2026-05-17  
+**Status:** In progress — 0015 ✅, 0016 ✅
+**Produced by:** grill-with-docs session, 2026-05-17
 **Prerequisite:** AGENTS.md refactor issue must be completed before skill implementation begins
 
 ---