docs: add PR quality contract

[BenSheridanEdwards]

Why does this feature exist?

Agents need a repository-local quality contract so completed work presents consistent implementation evidence at review time.

What changed?

• Added .github/pull_request_template.md with the required review sections.
• Added .agents/definition_of_done.md defining completed-PR requirements.
• Added .agents/skills/pr-quality-contract/SKILL.md so agents have a repository-local process for this gate.

Behavioural Proof (with video and screenshots)

• Video: Not applicable; this is a repository governance/docs change with no runtime UI behavior.
• Screenshots: Not applicable; this is a repository governance/docs change with no runtime UI behavior.
• Behaviour tests: python3 .agents/verify_quality_contract.py <worktree> run locally before commit; verifier was not committed.
• E2E mapping: Not applicable; no user-facing flow changed.

Verification Summary

• Definition of Done: Satisfied for a docs-only governance PR; unavailable UI proof is marked not applicable with reason.
• Commands run:
  • git diff --check
  • python3 .agents/verify_quality_contract.py <worktree>
• Results:
  • git diff --check: passed
  • quality contract verifier: quality contract verifier passed
• Known risks or skipped checks: Full product test suites were not run because this PR only changes PR governance docs and agent instructions.

[github-actions]

Fallow audit report

Found 1 finding.

Details

Severity	Rule	Location	Description
major	fallow/unused-dependency	packages/cli/package.json:17	Package '@galaxy-graph/core' is in dependencies but never imported; imported in other workspaces: examples/basic, packages/adapters

Generated by fallow.

[github-actions]

Fallow audit report

0 inline findings selected for GitHub review.