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/sharing-skills/SKILL.md

@@ -17,7 +17,7 @@ Contribute skills from your local branch back to the upstream repository.
 - Skill applies broadly (not project-specific)
 - Pattern/technique others would benefit from
 - Well-tested and documented
-- Follows skills/meta/writing-skills guidelines
+- Follows writing-skills guidelines
 
 **Keep personal when:**
 - Project-specific or organization-specific
@@ -29,7 +29,7 @@ Contribute skills from your local branch back to the upstream repository.
 
 - `gh` CLI installed and authenticated
 - Working directory is `~/.config/superpowers/skills/` (your local clone)
-- Skill has been tested (see skills/meta/writing-skills for TDD process)
+- **REQUIRED:** Skill has been tested using writing-skills TDD process
 
 ## Sharing Workflow
 
@@ -191,4 +191,4 @@ Each skill should:
 
 ## Related Skills
 
-- **skills/meta/writing-skills** - How to create well-tested skills
+- **writing-skills** - REQUIRED: How to create well-tested skills before sharing