feat(agents): add thinking model guidance reference files (#1722) (#1820)

Combines implementation by @davesienkowski (inline @-reference wiring at
decision-point steps, named reasoning models with anti-patterns, sequencing
rules, Gap Closure Mode) and @Tibsfox (test suite covering file existence,
section structure, and agent wiring).

- 5 reference files in get-shit-done/references/ — each with named reasoning
  models, Counters annotations, Conflict Resolution sequencing, and When NOT
  to Think guidance
- Inline @-reference wiring placed inside the specific step/section blocks
  where thinking decisions occur (not at top-of-agent)
- Planning cluster includes Gap Closure Mode root-cause check section
- Test suite: 63 tests covering file existence, named models, Conflict
  Resolution sections, Gap Closure Mode, and inline wiring placement

Closes #1722

Co-authored-by: Tibsfox <tibsfox@users.noreply.github.com>
Co-authored-by: Rezolv <davesienkowski@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

agents/gsd-debugger.md

@@ -957,6 +957,9 @@ Gather symptoms through questioning. Update file after EACH answer.
 </step>
 
 <step name="investigation_loop">
+At investigation decision points, apply structured reasoning:
+@~/.claude/get-shit-done/references/thinking-models-debug.md
+
 **Autonomous investigation. Update file continuously.**
 
 **Phase 0: Check knowledge base**