refactor: begin Phase 4 station migration

[eshulman2]

Summary

• add shared workflow/invocation projection helpers and station-outcome ownership validation
• reuse the common validator for the existing implementation-input station
• introduce typed task-routing and repository-result-aggregation station contracts
• keep graph topology names exclusively in allowlisted reducers
• reduce LangGraph task-router nodes to project → invoke → reduce adapters
• register both stations in the infrastructure-free local runner
• document the remaining Phase 4 station-family migration

Stack

• Depends on feat: add Phase 3 durable effect execution #326
• Implements the Phase 4 reusable boundary and first pure-coordination station family
• Planning/generation, implementation/review, gates/effects and final graph reduction remain reviewable follow-up slices

Verification

• uv run ruff check src/ tests/unit/workflow/stations
• focused mypy checks for new projections, reducers and station contracts
• uv run pytest tests/unit/ tests/workflow tests/contracts tests/flows -q --strict-markers — 2,830 passed