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/php/SKILL.md

@@ -0,0 +1,90 @@
+---
+name: php
+description: Builds PHP code, database migrations, and PHPUnit tests
+category: development
+---
+
+# PHP Developer
+
+Builds PHP code, database migrations, and tests for a feature.
+
+## When to Use
+
+```
+/php features/PROJ-X-feature-name.md
+```
+
+## Skill Steps
+
+1. **Read context**
+   - Read `features/INDEX.md` for feature overview
+   - Read the feature spec for requirements and tech design
+   - Check existing structure: `ls src/`, `ls tests/`
+
+2. **Implement PHP code**
+   - Create/update classes in `src/` directory
+   - Follow PSR-12 coding standard
+   - Use typed properties and return types
+   - Add `declare(strict_types=1);` to all PHP files
+
+3. **Implement migrations**
+   - Add migration file in `migrations/` directory
+   - Use portable SQL (see `docs/production/database-portability.md`)
+   - Use `IF NOT EXISTS` for idempotency
+
+4. **Write tests**
+   - PHPUnit tests in `tests/` directory
+   - Test one thing per method
+   - Test both happy path and error paths
+
+5. **Run checks**
+   - `composer lint` (fix any style issues)
+   - `composer analyze` (fix any static analysis errors)
+   - `composer test` (all tests must pass)
+
+6. **Update tracking**
+   - Mark feature as "Implemented" in `features/INDEX.md`
+
+7. **Suggest next step**
+   - Recommend `/qa` next
+
+## Tech Design Section Format (for feature specs)
+
+```markdown
+## Tech Design
+
+### Classes
+- `UserRepository`: handles user CRUD operations
+- `UserService`: business logic for users
+
+### Database Schema
+```sql
+CREATE TABLE users (
+    id INTEGER PRIMARY KEY,
+    email TEXT NOT NULL UNIQUE,
+    name TEXT NOT NULL,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+```
+
+### Implementation Order
+1. Create migration
+2. Implement UserRepository
+3. Implement UserService
+4. Write tests
+```
+
+## Rules
+
+- **PSR-12**: All code follows PSR-12 coding standard
+- **Strict types**: All files start with `declare(strict_types=1);`
+- **Parameterized queries**: Never interpolate user input into SQL
+- **Portable SQL**: Works on SQLite, MariaDB, and PostgreSQL
+- **Tests**: Write tests for all new classes/methods
+
+## Context Recovery
+
+If context is compacted:
+1. Re-read feature spec
+2. Check `git status` for uncommitted changes
+3. Continue from next class/method to implement