feat(governance): implement ADR-003 enforcement hooks (commit-msg, branch naming, pickup-issue skill)

## Summary

Implement the progressive enforcement mechanisms defined in ADR-003's enforcement table (added in PR #144).

## Tasks

- [ ] **Commit-msg hook** (Tier 0) — Reject commits without `Refs #N` or `Fixes #N` in the message
- [ ] **Branch naming validation** (pre-push) — Validate branches match `(feat|fix|chore|docs)/-*`
- [ ] **`pickup-issue` skill** (Claude Code plugin) — Agent must invoke before implementation; hard-fails without valid issue
- [ ] **Decompose ADR-003** into Guide + Skills (Phase 2 of ADR-012 migration plan)

## Context

These were identified as "follow-up work after PR #144 merges" in that PR's comment thread. ADR-003 documents the enforcement table; this issue tracks the implementation.

## References

- ADR-003 §Enforcement mechanisms (docs/decisions/003-contribution-governance.md)
- ADR-012 §Layer 3 — executable skills
- PR #144 comment (follow-up work list)