feat: add /gsd:debug for systematic debugging with persistent state
- Creates .planning/debug/[slug].md immediately on command
- Continuous updates: file IS the debugging brain
- Clear section rules: OVERWRITE (Current Focus, Resolution) vs APPEND (Evidence, Eliminated)
- Survives /clear perfectly - resume from any interruption point
- Scientific method: evidence → hypothesis → test → eliminate or confirm
- Includes debugging reference files (mindset, hypothesis testing, investigation techniques)
Closes #53, closes #44
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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