Implement Flow

Build from one plan file inside a goal workspace. Follow /orchestrate-flow.

Preconditions

1. Resolve goal-id and plan file (plans/NN-*.md or frontier from plans/INDEX.md)
2. Read GOAL.md, GRILL.md (if any), that plan file, plans/INDEX.md, and .agents/temp/grills/{language,choice,rules}.md when present
3. /taste-flow (+ /design-flow if UI); ticket brief via /trackers-flow if needed
4. REGISTRY: serialize if another running goal shares File lane

Process