Introduce session-level state management for tools that need to maintain
state across conversation turns within the same session (UMO).
- Add ToolSessionManager class for central per-(UMO, tool_name) state
- Add ToolSessionState with set_persistent(key) support
- Add is_stateful flag to FunctionTool base class
- Wire session_manager through run_context in all agent runners
- Update ExecuteShellTool to use framework session manager
- Update CLAUDE.md with stateful tool documentation
BREAKING: Tools that were manually managing session state via _sessions
dict should migrate to use the framework's ToolSessionManager for proper
lifecycle management and persistence support.
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