refactor: remove domain expertise feature

The ~/.claude/skills/expertise/ system was personal tooling
that doesn't exist for other users. Removed from:
- create-roadmap workflow (detect_domain step)
- plan-phase workflow (domain loading)
- roadmap template (Domain Expertise section)
- phase-prompt template (domain frontmatter)
- plan-format reference (domain field)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/gsd/create-roadmap.md

@@ -66,11 +66,10 @@ If "Replace": Continue with workflow
 </step>
 
 <step name="create_roadmap">
-Follow the create-roadmap.md workflow starting from detect_domain step.
+Follow the create-roadmap.md workflow starting from identify_phases step.
 
 The workflow handles:
 - Loading requirements
-- Domain expertise detection
 - Phase identification mapped to requirements
 - Requirement coverage validation (no orphaned requirements)
 - Research flags for each phase