Consume snapshotPhaseInput in runProviderPhase

[VincentShipsIt]

Parent: #126

What

Change runProviderPhase signature to accept PhaseInput (frozen read-only snapshot) instead of full mutable PipelineContext. Providers can no longer mutate orchestrator state.

Scope

• packages/pipeline/src/pipeline/runtime.ts — runProviderPhase signature change
• All callers of runProviderPhase in planning-phases.ts and execution-phases.ts — call snapshotPhaseInput(context) before passing
• Post-call mutations (telemetry push, model resolution) stay on orchestrator side via return value

Estimate

~30 lines. Low risk — PhaseInput and snapshotPhaseInput already exist and are exported.

Depends on

Nothing — can be done independently.