fix(ml_debug): address review feedback

- Fix stale Part 2 cross-references to link to rl/SKILL.md
- Add McCandlish + Slavv back to parent Sources (cited in Part 7)
- Add back-links from refs/ files to parent SKILL.md

refs/diagnostics.md

@@ -1,5 +1,7 @@
 # 6.2 Diagnostic code snippets
 
+Part of the [ML Debugging skill](../SKILL.md), section 6.2.
+
 Copy-paste these. Each tests one thing.
 
 **Data pipeline sanity check**

refs/static_analysis.md

@@ -1,5 +1,7 @@
 # 6.1 Static analysis: grep for silent bugs
 
+Part of the [ML Debugging skill](../SKILL.md), section 6.1.
+
 Run these searches on the codebase before anything else. Each catches a common bug that produces no error but wrong results.
 
 **Shape mismatches (silent broadcasting)**