The working_stack tool was defined in tools/mod.rs but implemented
in agent.rs as Agent::handle_working_stack(). This orphaned the tool
from the rest of the tool infrastructure.
Move the implementation to tools/working_stack.rs so it follows the
same pattern as other tools. The tool still needs special handling
in agent.rs because it requires mutable access to context state,
but the implementation is now in the right place.
Changes:
- Created tools/working_stack.rs with handle() and format_stack()
- Updated tools/mod.rs to use working_stack::definition()
- Removed handle_working_stack() and format_stack() from Agent
- Agent now calls tools::working_stack::handle() directly
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