Step 3: Extract AntiStallMonitor from run_stream() ...

- Create navi/core/anti_stall.py with AntiStallMonitor class
- Encapsulates stall detection (todo progress + repeated tool calls)
- Encapsulates adaptive re-plan (failed todo step detection)
- Provides init() / pre_turn() / post_turn() two-phase interface
- Remove ~50 lines of stall/replan logic from agent.py run_stream()
- Remove _todo_status_snapshot and _todo_failed_steps helpers from agent.py
- Update AgentTurnContext: remove stall fields (now live in AntiStallMonitor)
- Add 13 unit tests for pre_turn and post_turn behavior

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

Step 2: Extract AgentTurnContext dataclass from run_stream() ...

Move 10 turn-level local variables from run_stream() into AgentTurnContext:
- turn_start, tool_call_count, turn_tokens, subagent_tokens
- stall_no_todo, stall_repeat_tools, prev_tool_sigs
- known_failed, replan_msg, injected_fact_ids

This makes run_stream() readable and prepares the ground for
AntiStallMonitor (Step 3) which will consume this context.

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