Per-phase context factory: fresh context per invocation

[VincentShipsIt]

Parent: #126

What

Each phase invocation gets a fresh PhasePayload built by a factory function. No shared mutation between phases. Factory takes orchestrator state + previous phase output and produces a minimal, scoped context.

function buildPhasePayload(
  state: OrchestratorState,
  previousOutput: PhaseOutcome,
  materials: PromptMaterial[],
): PhasePayload

Scope

• packages/pipeline/src/pipeline/context.ts — buildPhasePayload factory
• Integration with dispatch loop from Add CLI command coverage and retry routing #132

Estimate

~100 lines. Medium risk — depends on #131 and #132 landing clean.

Depends on

#131 (split context), #132 (dispatch loop).