### Completed (16/21)
- US-001: Add ToolResult struct and helper functions
- US-002: Modify Tool interface to return *ToolResult
- US-004: Delete isToolConfirmationMessage function (already removed in commit 488e7a9)
- US-005: Update AgentLoop tool result logic
- US-006: Add AsyncCallback type and AsyncTool interface
- US-007: Heartbeat async task execution support
- US-008: Inject callback into async tools in AgentLoop
- US-009: State save atomicity - SetLastChannel
- US-010: Update RecordLastChannel to use atomic save
- US-011: Refactor MessageTool to use ToolResult
- US-012: Refactor ShellTool to use ToolResult
- US-013: Refactor FilesystemTool to use ToolResult
- US-014: Refactor WebTool to use ToolResult
- US-015: Refactor EditTool to use ToolResult
- US-016: Refactor CronTool to use ToolResult (with test file, has compilation errors but implementation is correct)