v0.6.10: refactor: modularize executor and consolidate IG numbering

Extract executor.py into focused modules:
- act_wave_finalize.py: wave finalization and aggregation logic
- step_wave_types.py: step wave types and stream event handling
- thread_selection.py: thread routing and subagent wire selection
- tool_call_enrichment.py: tool call arg enrichment from chunks
- tool_call_id.py: unified tool call ID handling and subgraph binding

Remove obsolete modules:
- ephemeral_execute_stream.py (replaced by step_wave_types)
- fallback_summary.py (integrated into act_wave_finalize)
- tool_result_registry.py (subsumed by ledger-based tracking)

Consolidate IG numbering (625 series → 494-498, 480→485, etc.)
for cleaner sequential tracking. Add IG-493 for subagent fixes.