refactor(aidd-dev): lean the implement skill for small models #1076

Problem

aidd-dev:02-implement is 5 files and 732 words: three actions (prepare, execute, finalize). It reads phase-<n>.md beside plan.md and sets the plan status. The lean plan skill changes what it reads (an Impact section, a safety-net phase, documentation work carried as tasks), and nothing yet tells implement to apply them. Under opencode with Mistral small and medium models, medium has been seen to implement and commit when it was only asked to plan, so the boundary with plan must be explicit on both sides.

Scope

• Same method as the brainstorm and plan skills: mermaid router with action paths, recipe steps, one Output per action, rules only where the action owns them.
• Carry the plan-side contract: apply the documentation work already listed in the phases (feat(aidd-dev): carry documentation impact through feature delivery #927), run the safety-net baseline after every phase when the plan has one (feat(aidd-dev/plan): conditional Phase 0 "regression safety net" with regression risk assessment #896).
• Keep the status values it writes (in-progress, implemented, blocked), now documented in the plan template.
• Check the router on opencode with mistral-small-2603 and mistral-medium-2604.

Acceptance criteria

• The pre-commit gates pass, with fewer words than today.
• A plan written by the lean plan skill is implemented phase by phase on Claude haiku and sonnet and on mistral-medium-2604.
• The skill stops at the plan's acceptance criteria and does not commit or push unless asked.

Prior art in this repo

• The plan skill issue and feat(aidd-dev): carry documentation impact through feature delivery #927, feat(aidd-dev/plan): conditional Phase 0 "regression safety net" with regression risk assessment #896.
• plugins/aidd-dev/skills/09-for-sure, which also reads and sets the plan status.

Out of scope

• plan, assert, review: separate issues.
• auto mode of aidd-orchestrator:01-sdlc.