Improve skill cross-references for clarity and compliance

Update all skill references to use explicit requirement markers:
- REQUIRED BACKGROUND: For prerequisite understanding
- REQUIRED SUB-SKILL: For mandatory workflow dependencies
- Complementary skills: For optional but helpful related skills

Changes:
- Remove old path format (skills/collaboration/X → X)
- Add explicit "REQUIRED" markers to make dependencies clear
- Update Integration sections with categorized skill relationships
- Fix non-existent skill references
- Update cross-reference documentation in writing-skills

This makes it immediately clear which skills MUST be used vs optional references,
helping Claude understand and comply with skill dependencies.

skills/writing-plans/SKILL.md

@@ -33,7 +33,7 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 ```markdown
 # [Feature Name] Implementation Plan
 
-> **For Claude:** Use `${SUPERPOWERS_SKILLS_ROOT}/skills/collaboration/executing-plans/SKILL.md` to implement this plan task-by-task.
+> **For Claude:** REQUIRED SUB-SKILL: Use executing-plans to implement this plan task-by-task.
 
 **Goal:** [One sentence describing what this builds]
 
@@ -107,10 +107,10 @@ After saving the plan, offer execution choice:
 **Which approach?"**
 
 **If Subagent-Driven chosen:**
-- Use skills/collaboration/subagent-driven-development
+- **REQUIRED SUB-SKILL:** Use subagent-driven-development
 - Stay in this session
 - Fresh subagent per task + code review
 
 **If Parallel Session chosen:**
 - Guide them to open new session in worktree
-- New session uses skills/collaboration/executing-plans
+- **REQUIRED SUB-SKILL:** New session uses executing-plans