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/frontend/SKILL.md

@@ -0,0 +1,59 @@
+---
+name: frontend
+description: Builds UI components with React, Tailwind CSS, and shadcn/ui
+category: development
+---
+
+# Frontend Developer
+
+Builds the UI components for a feature using React, Next.js, Tailwind CSS, and shadcn/ui.
+
+## When to Use
+
+```
+/frontend features/PROJ-X-feature-name.md
+```
+
+## Skill Steps
+
+1. **Read context**
+   - Read `features/INDEX.md` for feature overview
+   - Read the feature spec for requirements
+   - Check existing components: `ls src/components/ui/`
+
+2. **Check shadcn/ui availability**
+   - List installed components in `src/components/ui/`
+   - Install missing with: `npx shadcn@latest add <component-name>`
+
+3. **Implement UI**
+   - Build components in `src/components/` and `src/app/`
+   - Use existing shadcn/ui components (NEVER recreate)
+   - Follow Tailwind CSS patterns
+   - Add TypeScript types
+
+4. **Write tests**
+   - Unit tests next to source files (`*.test.ts` next to `*.ts`)
+
+5. **Lint**
+   - Run `npm run lint` after changes
+
+6. **Update tracking**
+   - Mark feature as "Frontend Done" in `features/INDEX.md`
+
+7. **Suggest next step**
+   - Recommend `/backend` or `/qa` next
+
+## Rules
+
+- **shadcn/ui first**: Always check `src/components/ui/` before creating new components
+- **Tailwind CSS**: Use utility classes, never raw CSS
+- **TypeScript**: All components must be typed
+- **Tests**: Write unit tests for all components
+- **Lint**: Run lint after every change
+
+## Context Recovery
+
+If context is compacted:
+1. Re-read feature spec
+2. Check `git diff` for uncommitted changes
+3. Continue implementation