fix(apodex): offload per-turn session persist off the event loop

[Ray0907]

Summary

• TerminalSession._on_turn fires after every agent turn (run_agent_loop awaits on_turn_complete directly) and called _persist() inline — a synchronous open() + json.dump() over the full history, display_history, and workflow_turns.
• As a session grows, this write grows with it, and being awaited directly on the hot per-turn path it stalls the event loop every single turn — no other coroutine (TUI redraw, streaming, etc.) gets to run during the write.
• Moved it to asyncio.to_thread, still awaited so writes stay ordered turn-to-turn (no risk of an out-of-order write corrupting the --resume checkpoint).

Benchmark

2000-message history, 10 persists, with a concurrent heartbeat coroutine standing in for other async work:

sync (current)         elapsed=139.1ms  heartbeat_ticks=   0  max_stall=0.00ms
asyncio.to_thread      elapsed=195.4ms  heartbeat_ticks=4558  max_stall=0.85ms

Sync blocks the loop completely (0 heartbeat ticks in 139ms). to_thread costs a bit more wall time (thread dispatch + GIL handoff) but lets everything else keep running concurrently — the actual user-visible win, since the write itself isn't the bottleneck, the freeze is.

Test plan

• uv run pytest apodex/tests/test_changes.py apodex/tests/test_features.py — 136 passed
• uv run ruff check apodex/session.py — clean