refactor: clean up tool dispatch and extract helpers
- Move working_stack tool to tools/working_stack.rs (was orphaned in agent.rs)
- Create control.rs for pause/switch_model/yield_to_user with Result<ToolOutput>
- Add ToolOutput::error() and ToolOutput::text() helper constructors
- Clean up dispatch() with Option<Result<ToolOutput>> pattern for rich tools
- Refactor memory.rs: extract cmd(), write_node(), supersede(), get_str(), get_f64()
- Merge run_rg() and run_grep() into unified run_search() in grep.rs
- Extract truncate_output() helper shared by bash, grep, glob tools
Net: -77 lines, better structure, less duplication
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