Extract PlanningEngine, ContextBuilder, ToolExecutor from agent.py ...

- navi/core/planning.py: new 3-phase planning pipeline (~390 lines)
- navi/core/context_builder.py: system prompt caching, memory/context injection, goal anchoring (~160 lines)
- navi/core/tool_executor.py: tool execution with middleware chain (~150 lines)
- navi/core/agent.py: reduced from ~1420 to ~770 lines; delegates to extracted classes

All compilation verified.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>