refactor(skills): fold systematic-debugging Related-skills block into Phase 4

Same treatment as subagent-driven-development and executing-plans: the
test-driven-development entry duplicated the reference already at Phase 4
Step 1, and the verification-before-completion entry was a sole carrier —
it moves to its point of use in Phase 4 Step 3 (Verify Fix). Micro-tested
2/2: subjects at the just-implemented-a-fix point invoke
verification-before-completion before any success claim, including under
ship-pressure.

skills/systematic-debugging/SKILL.md

@@ -188,6 +188,7 @@ You MUST complete each phase before proceeding to the next.
    - Test passes now?
    - No other tests broken?
    - Issue actually resolved?
+   - Use the `superpowers:verification-before-completion` skill before claiming success
 
 4. **If Fix Doesn't Work**
    - STOP
@@ -283,10 +284,6 @@ These techniques are part of systematic debugging and available in this director
 - **`defense-in-depth.md`** - Add validation at multiple layers after finding root cause
 - **`condition-based-waiting.md`** - Replace arbitrary timeouts with condition polling
 
-**Related skills:**
-- **superpowers:test-driven-development** - For creating failing test case (Phase 4, Step 1)
-- **superpowers:verification-before-completion** - Verify fix worked before claiming success
-
 ## Real-World Impact
 
 From debugging sessions: