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/architecture/SKILL.md

@@ -0,0 +1,70 @@
+---
+name: architecture
+description: Designs technical architecture for feature specs
+category: development
+---
+
+# Solution Architect
+
+Designs the technical architecture for a feature and adds it to the feature spec.
+
+## When to Use
+
+Run this skill after `/requirements` to design the technical approach:
+
+```
+/architecture features/PROJ-X-feature-name.md
+```
+
+## Skill Steps
+
+1. **Read feature spec**
+   - Read the feature spec file created by `/requirements`
+
+2. **Design architecture**
+   - **Component hierarchy**: List all components needed
+   - **API design**: Endpoints (method, path, request/response shapes)
+   - **Database schema**: Tables, columns, RLS policies (if Supabase)
+   - **Implementation order**: What to build first, dependencies
+
+3. **Add Tech Design section**
+   - Append "Tech Design" section to the feature spec
+
+4. **Update tracking**
+   - Update `features/INDEX.md` status to "Designed"
+
+5. **Suggest next step**
+   - Recommend running `/frontend` or `/backend` next
+
+## Tech Design Section Format
+
+```markdown
+## Tech Design
+
+### Components
+- ComponentName: description, props
+
+### API Endpoints
+| Method | Path | Request | Response |
+|--------|------|---------|----------|
+| POST | /api/entities | {...} | {id, ...} |
+
+### Database Schema
+- Table: entities
+  - id: uuid (PK)
+  - name: text
+  - created_at: timestamp
+
+### Implementation Order
+1. Create database schema
+2. Build API endpoints
+3. Build UI components
+4. Add tests
+```
+
+## Context Recovery
+
+If context is compacted:
+1. Re-read the feature spec
+2. Check if Tech Design section already exists
+3. Continue from implementation order