Improve Copilot contributor guidance and request scaffolding

[Kane610]

Summary

• deduplicate and expand Copilot project instructions for higher-signal requests
• add structured templates for Copilot asks and PR submissions
• add contributor quick start for common workflows and pitfalls
• clarify when to use Axis review agents

What Changed

• updated .github/copilot-instructions.md
  • removed duplicated Project Guidelines section
  • added high-quality request template
  • added code review request templates (review-only vs verify)
  • added common failure modes and troubleshooting guidance
• added .github/pull_request_template.md
• added .github/ISSUE_TEMPLATE/copilot-ask.md
• added COPILOT_QUICK_START.md
• updated .github/agents/axis-review.agent.md with "When to Use This Agent"
• updated .github/agents/axis-review-verify.agent.md with "When to Use This Agent"

Validation

• uv run ruff check axis tests
• uv run ruff format --check axis tests
• uv run mypy axis
• uv run pytest

All passed locally (370 passed, coverage 97.89%).

Notes

• intentionally removed obsolete tests/respx_shim.py from local working tree before commit (not included in this PR)