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

[scottschreckengaust]

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)/<issue-number>-*
• 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 docs: ADR-005 through ADR-013 (feedback, flags, knowledge, DoD, security, recovery, conflicts, knowledge stack, validation pyramid) #144 comment (follow-up work list)