fix(agent): stop passive mode when idle

[HelloAnner]

Summary

• make passive mode wait before calling the model when startup or restored context has no pending work
• suppress autonomous heartbeat injection while passive mode is enabled
• return to passive rest immediately after an event ends without tool calls
• preserve pending tool-result continuation before resting

Validation

• uv run --frozen python scripts/check_version.py
• uv run --frozen ruff check src tests scripts apps/explore-lab/backend
• uv run --frozen mypy src
• uv run --frozen pytest (1450 passed)

Risks and compatibility

• Behavior changes only when agent.passive_mode=true.
• Active mode heartbeat and timed rest behavior are unchanged.
• Passive tool chains still continue while a tool result needs model processing, then rest after completion.
• This reduces unintended background model usage; it does not change provider, API, storage, or communication formats.

Checks not run

• Rust and frontend checks were not run because this change only touches the Python agent loop and unit tests.

[Zoupers]

初步想了一下，这个好像是会影响模型的启动的，例如我们现在刚刚初始化，配置完模型，然后系统重启了
这个时候模型相当于首次启动，其short_term应该为空，那这个时候好像就会直接停住，而不是模型自己探索，这个是这个PR所期望的行为嘛？

[HelloAnner]

等待批复 ing