Prefer documented verification commands in ACI suggestions

[northline-lab]

Summary

Add logic so aci_suggest_verification prefers repository-documented verification commands from CONTRIBUTING.md over generic fallbacks, and add a unit test covering this behavior.

Verification

• Unit tests could not be executed with pytest due to missing dev deps (pydantic) and uv not being available in this environment. This is expected per repo docs requiring uv for dev extras. As a minimal check, compiled the repo with python3 -m compileall -q . successfully.
• Used aci_submit_patch successfully to capture the diff. The patch includes code changes and a new unit test.
• aci_suggest_verification before patch returned generic Python suggestions; after changes, logic will include commands extracted from CONTRIBUTING.md first.
• Blocking details recorded below.

Risk

• Selected task risk: low
• Files changed: src/contribarena/tools/aci.py, tests/unit/test_aci_tools.py

Dry-Run Notice

This is a dry-run PR draft. No external repository write was performed.