Clamp read_file limits across agents

[Gujiassh]

Summary

• clamp read_file limits to non-negative values and honor limit=0 across agents
• update read_file signatures to accept optional limit values consistently

Testing

• python3 -m py_compile agents/s02_tool_use.py agents/s03_todo_write.py agents/s04_subagent.py agents/s05_skill_loading.py agents/s06_context_compact.py agents/s07_task_system.py agents/s08_background_tasks.py agents/s09_agent_teams.py agents/s10_team_protocols.py agents/s11_autonomous_agents.py agents/s12_worktree_task_isolation.py agents/s_full.py

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