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/rules/database.md

@@ -0,0 +1,80 @@
+# Database Rules
+
+SQLite for development, MariaDB/PostgreSQL for production. Write portable SQL.
+
+## Development Database
+
+- Use SQLite for local development (zero-config, no server)
+- Database file: `database.sqlite` in project root
+- Create with: `touch database.sqlite`
+
+## SQL Portability
+
+### Use Standard SQL Only
+
+- `CURRENT_TIMESTAMP` — works on SQLite, MariaDB, PostgreSQL
+- `INTEGER PRIMARY KEY` — portable (mapped to SERIAL/BIGSERIAL)
+- `TEXT` — portable (avoid VARCHAR length constraints)
+- `DATETIME` — portable across all three databases
+
+### Avoid
+
+| Pattern | Why | Alternative |
+|---------|-----|-------------|
+| `REPLACE INTO` | Deletes then inserts (data loss) | `UPDATE ...; INSERT IF 0 rows affected` |
+| `NOW()` | SQLite/MariaDB specific | `CURRENT_TIMESTAMP` |
+| `GROUP_CONCAT()` | Not in SQLite | Application-level aggregation |
+| `IFNULL()` | SQLite syntax | `COALESCE()` (standard) |
+| Database-specific functions | Locks you in | Do it in application code |
+
+### Boolean Fields
+
+SQLite has no BOOLEAN — use INTEGER (0/1):
+
+```sql
+CREATE TABLE settings (
+    dark_mode INTEGER DEFAULT 0,
+    notifications INTEGER DEFAULT 1
+);
+```
+
+### String Concatenation
+
+Do string concatenation in PHP, not SQL.
+
+### Upsert Pattern
+
+```php
+// Safe upsert (works on all databases)
+$affected = $pdo->prepare(
+    'UPDATE users SET name = ? WHERE id = ?'
+)->execute([$name, $id]);
+
+if ($affected === 0) {
+    $pdo->prepare(
+        'INSERT INTO users (id, name) VALUES (?, ?)'
+    )->execute([$id, $name]);
+}
+```
+
+## Migrations
+
+- Store migrations in `migrations/` directory
+- Name format: `001_description.sql`, `002_description.sql`
+- Each migration must be idempotent (use `IF NOT EXISTS`)
+- Run migrations on every environment
+
+## Indexes
+
+- Index columns used in `WHERE`, `JOIN`, `ORDER BY`
+- Composite indexes: put most selective column first
+- Don't over-index (write performance cost)
+
+## No N+1 Queries
+
+- Use JOINs or batch queries
+- Eager load relationships when needed
+
+## Transactions
+
+Use transactions for any operation that modifies more than one table or row.