Unify OpenCode and Codex starter kits

- Add .shared/prompts/ with tool-neutral prompts
- Add .opencode/config/project.md, rules/, skills/ from opencode-kit
- Add .codex/config/project.md from codex-kit
- Copy .shared/prompts/ to both .opencode/prompts/ and .codex/prompts/
- Add features/INDEX.md and features/README.md
- Update README with unified structure

The two original starter kits are now redundant.

.opencode/skills/backend/SKILL.md

@@ -0,0 +1,64 @@
+---
+name: backend
+description: Builds APIs, database schemas, and RLS policies with Supabase
+category: development
+---
+
+# Backend Developer
+
+Builds API endpoints, database schemas, and RLS policies for a feature.
+
+## When to Use
+
+```
+/backend features/PROJ-X-feature-name.md
+```
+
+## Skill Steps
+
+1. **Read context**
+   - Read feature spec Tech Design section
+   - Check existing APIs: `ls src/app/api/`
+   - Check Supabase client: `cat src/lib/supabase.ts`
+
+2. **Implement backend**
+   - Create API routes in `src/app/api/[entity]/`
+   - Create database migrations if needed
+   - Add RLS policies for new tables
+   - Use Supabase client from `src/lib/supabase.ts`
+
+3. **Write tests**
+   - Unit tests for all endpoints
+   - Integration tests for API routes
+
+4. **Verify**
+   - Run `npm test` to ensure tests pass
+
+5. **Update tracking**
+   - Mark feature as "Backend Done" in `features/INDEX.md`
+
+6. **Suggest next step**
+   - Recommend `/qa` next
+
+## API Route Pattern
+
+```
+src/app/api/[entity]/
+├── route.ts          # GET, POST
+├── [id]/route.ts     # GET, PUT, DELETE
+```
+
+## Rules
+
+- Use Supabase client from `src/lib/supabase.ts`
+- Follow existing route patterns
+- Add RLS policies for all new tables
+- Write tests for all endpoints
+- Run `npm test` after implementation
+
+## Context Recovery
+
+If context is compacted:
+1. Re-read feature spec Tech Design
+2. Check `git diff` for uncommitted changes
+3. Continue from next endpoint