feat(13-01): create gsd-debugger agent with consolidated debugging expertise
- All 12 sections present: role, philosophy, hypothesis_testing,
investigation_techniques, verification_patterns, research_vs_reasoning,
debug_file_protocol, execution_flow, checkpoint_behavior,
structured_returns, modes, success_criteria
- Consolidated ~2,400 lines to 990 lines (59% reduction)
- Complete debugging methodology: scientific method, meta-debugging,
cognitive biases, investigation techniques, verification patterns
- Full checkpoint and return format specifications
- Debug file protocol with update rules and status transitions
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