refactor(flow): replace Phase 5 step with Cadre subflow primitive

[jafreck]

Summary

Replace the manual FlowRunner.run() inside executeIterativeMigration() with the framework's declarative subflow() node from @cadre-dev/framework. Phase 5's iterative migration is now expressed as a first-class child flow in the pipeline DSL rather than an opaque step that internally spawns its own runner.

Motivation

executeIterativeMigration was a step() that internally built a FlowDefinition, instantiated its own FlowRunner, and managed checkpoint/concurrency manually — exactly the pattern subflow() was designed to replace. With subflow(), the parent flow's runner manages the child flow lifecycle, checkpoint propagation, abort signal forwarding, and error handling automatically.

Changes

src/flow/migration-flow.ts

• Replace step() for 'iterative-migration' with subflow() using a flow thunk
• The thunk dynamically builds the child flow via buildPhase5Subflow() and populates runner options (checkpoint adapter + concurrency) before execution
• contextMap identity-maps MigrationFlowContext (same type for parent and child)
• Remove unused fromStep, PhaseResult imports

src/flow/steps/migration.ts

• Extract discoverTasks() and sortTasksSccAware() as internal helpers
• Export buildPhase5Subflow() — task discovery, SCC-aware sort, cost projection, and child flow construction (per-task or wave-barrier)
• Export computePhase5Concurrency() and computePhase5RunnerOptions()
• Delete executeIterativeMigration() — no longer needed

Tests

• Replace all executeIterativeMigration() calls with local runPhase5() helpers that call buildPhase5Subflow() + FlowRunner.run() directly
• Update assertions from PhaseResult (.success / .phase) to FlowRunResult (.status)

Verification

• tsc --noEmit — clean
• 254/254 flow tests pass
• Zero remaining references to executeIterativeMigration in the codebase