refactor(projman): normalize RFC to sub-command pattern, absorb clear-cache

- Created unified /rfc command with create|list|review|approve|reject sub-commands
- Deleted 5 individual rfc-*.md command files
- Moved /clear-cache into /setup --clear-cache
- Updated all cross-references in skills, docs, and integration files
- Command count: 17 -> 12 (net -5)

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

README.md

@@ -19,7 +19,7 @@ AI-guided sprint planning with full Gitea integration. Transforms a proven 15-sp
 - Branch-aware security (development/staging/production)
 - Pre-sprint-close code quality review and test verification
 
-**Commands:** `/sprint-plan`, `/sprint-start`, `/sprint-status`, `/sprint-close`, `/labels-sync`, `/setup`, `/review`, `/test`, `/debug`, `/suggest-version`, `/proposal-status`, `/clear-cache`, `/rfc-create`, `/rfc-list`, `/rfc-review`, `/rfc-approve`, `/rfc-reject`
+**Commands:** `/sprint-plan`, `/sprint-start`, `/sprint-status`, `/sprint-close`, `/labels-sync`, `/setup`, `/review`, `/test`, `/debug`, `/suggest-version`, `/proposal-status`, `/rfc`
 
 #### [git-flow](./plugins/git-flow) *NEW in v3.0.0*
 **Git Workflow Automation**