Split PipelineContext into orchestrator state and phase payload

[VincentShipsIt]

Parent: #126

What

Separate PipelineContext into two structs:

• OrchestratorState — retry counters, turn count, telemetry arrays, abort controller, phase routing config, model assignments. Owned by orchestrator, persists across phases.
• PhasePayload — prompt materials, repo context, worktree path, model hints, reasoning effort. Built fresh per phase invocation from orchestrator state + previous phase output.

Scope

• packages/pipeline/src/types.ts — new OrchestratorState and PhasePayload types
• packages/pipeline/src/pipeline/context.ts — factory function buildPhasePayload(state, prevOutput)
• packages/pipeline/src/pipeline/runtime.ts — runProviderPhase takes PhasePayload
• packages/pipeline/src/pipeline/planning-phases.ts / execution-phases.ts — read from payload, not context

Estimate

~200 lines. Medium risk — touches core types used everywhere.

Depends on

#130 (consume snapshotPhaseInput) should land first.