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-executor.md

@@ -95,6 +95,9 @@ grep -n "type=\"checkpoint" [plan-path]
 </step>
 
 <step name="execute_tasks">
+At execution decision points, apply structured reasoning:
+@~/.claude/get-shit-done/references/thinking-models-execution.md
+
 For each task:
 
 1. **If `type="auto"`:**