Reset phase-specific context fields between pipeline transitions

[VincentShipsIt]

Parent: #126

Problem

Phase-specific fields on PipelineContext (e.g., stabilizationFeedback, testOutput, previousPlanRawOutput) persist across transitions. Some are manually nulled, but there's no systematic reset — fields from phase N can leak into phase N+1.

Solution

Add resetPhaseState(context, nextPhase) call at each phase transition point. Clears fields that are phase-scoped:

• stabilizationFeedback → only for verification
• testOutput → only for testing/test-fix
• previousPlanRawOutput → only for revision
• Other phase-local fields identified during implementation

Files

• packages/pipeline/src/pipeline/context.ts — new resetPhaseState function
• packages/pipeline/src/pipeline/planning-phases.ts — call at transitions
• packages/pipeline/src/pipeline/execution-phases.ts — call at transitions
• packages/pipeline/src/types.ts — document which fields are phase-scoped

[VincentShipsIt]

Shipped in develop. resetPhaseState() called at all phase transitions in planning-phases.ts and execution-phases.ts.