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/issues/0020-design-skills.md

@@ -1,6 +1,6 @@
 # 0020 — Design skills: grill-lean, grill-me, write-prd, architecture-review, break-into-issues, prototype
 
-**Type:** HITL  
+**Type:** HITL
 **Parent PRD:** `docs/prd/chunk-3-skills-library.md`
 
 ## What to build