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
 
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docs/prd/governance-instruction-layer.md

@@ -1,8 +1,8 @@
 # PRD: Governance Instruction Layer (Phase 1)
 
-**Workstream:** Governance (parallel, not a numbered chunk)  
-**Phase:** 1 of 2 — instruction and documentation layer  
-**Must complete before:** Chunk 3  
+**Workstream:** Governance (parallel, not a numbered chunk)
+**Phase:** 1 of 2 — instruction and documentation layer
+**Must complete before:** Chunk 3
 **Phase 2 spec:** `docs/research/governance_principles/CONTROLS.md` — deferred to Chunk 6
 
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