feat: feature impl orchestration

[robhowley]

• Encodes a behavior-first, minimal-patch implementation discipline as a skill
• Forces intent → behavior definition → tests → code → proof, in that order
• Prevents AI overbuilding: no speculative abstractions, no scope creep, no "looks correct" completions
• Automatically delegates to relevant domain skills (fastapi, sqlalchemy, etc.) as needed
• Works across any non-trivial feature implementation in a Python/FastAPI project