feat(references): add common bug patterns checklist for debugger agent (#1780)

* feat(references): add common bug patterns checklist for debugger

Create a technology-agnostic reference of ~80%-coverage bug patterns
ordered by frequency — off-by-one, null access, async timing, state
management, imports, environment, data shape, strings, filesystem,
and error handling. The debugger agent now reads this checklist before
forming hypotheses, reducing the chance of overlooking common causes.

Closes #1746

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(references): use bold bullet format in bug patterns per GSD convention (#1746)

- Convert checklist items from '- [ ]' checkbox format to '- **label** —'
  bold bullet format matching other GSD reference files
- Scope test to <patterns> block only so <usage> section doesn't fail
  the bold-bullet assertion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

agents/gsd-debugger.md

@@ -31,6 +31,10 @@ If the prompt contains a `<files_to_read>` block, you MUST use the `Read` tool t
 - Handle checkpoints when user input is unavoidable
 </role>
 
+<required_reading>
+@~/.claude/get-shit-done/references/common-bug-patterns.md
+</required_reading>
+
 <philosophy>
 
 ## User = Reporter, Claude = Investigator