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>

README.md

@@ -344,6 +344,7 @@ You're never locked in. The system adapts.
 | `/gsd:consider-issues` | Review deferred issues, close resolved, identify urgent |
 | `/gsd:add-todo [desc]` | Capture idea or task from conversation for later |
 | `/gsd:check-todos [area]` | List pending todos, select one to work on |
+| `/gsd:debug [desc]` | Systematic debugging with persistent state across `/clear` |
 | `/gsd:help` | Show all commands and usage guide |
 
 <sup>¹ Contributed by reddit user OracleGreyBeard</sup>