ops: add preset config and CI policy gate

[flyingrobots]

Phase D2+D3 — Opinionated Starter + Policy Gate

Deliverables

• `git mind init --preset engineering`
• Default prefixes, edge types, confidence thresholds, doctor policies
• Strict mode: fail on dangling edges, blocked release-critical tasks, low-confidence edges
• GitHub Action docs/examples

Acceptance Criteria

• Sane defaults for 80% of teams
• Policy outcomes deterministic
• No false-green due to parsing/log ambiguity

Part of M9 — IRONCLAD GA

[flyingrobots]

Closing under the clarified product frame. This issue was tied to an older milestone-era GA/preset workflow and is not part of the current Hills-first operating model. If a policy gate becomes necessary later, it should be reframed against a current Hill with a sponsor user and playback evidence.