feat(agents): add database-reviewer agent with Supabase patterns (#48)

Adds comprehensive database-reviewer agent with Supabase PostgreSQL best practices

skills/postgres-patterns/SKILL.md

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+---
+name: postgres-patterns
+description: PostgreSQL database patterns for query optimization, schema design, indexing, and security. Based on Supabase best practices.
+---
+
+# PostgreSQL Patterns
+
+Quick reference for PostgreSQL best practices. For detailed guidance, use the `database-reviewer` agent.
+
+## When to Activate
+
+- Writing SQL queries or migrations
+- Designing database schemas
+- Troubleshooting slow queries
+- Implementing Row Level Security
+- Setting up connection pooling
+
+## Quick Reference
+
+### Index Cheat Sheet
+
+| Query Pattern | Index Type | Example |
+|--------------|------------|---------|
+| `WHERE col = value` | B-tree (default) | `CREATE INDEX idx ON t (col)` |
+| `WHERE col > value` | B-tree | `CREATE INDEX idx ON t (col)` |
+| `WHERE a = x AND b > y` | Composite | `CREATE INDEX idx ON t (a, b)` |
+| `WHERE jsonb @> '{}'` | GIN | `CREATE INDEX idx ON t USING gin (col)` |
+| `WHERE tsv @@ query` | GIN | `CREATE INDEX idx ON t USING gin (col)` |
+| Time-series ranges | BRIN | `CREATE INDEX idx ON t USING brin (col)` |
+
+### Data Type Quick Reference
+
+| Use Case | Correct Type | Avoid |
+|----------|-------------|-------|
+| IDs | `bigint` | `int`, random UUID |
+| Strings | `text` | `varchar(255)` |
+| Timestamps | `timestamptz` | `timestamp` |
+| Money | `numeric(10,2)` | `float` |
+| Flags | `boolean` | `varchar`, `int` |
+
+### Common Patterns
+
+**Composite Index Order:**
+```sql
+-- Equality columns first, then range columns
+CREATE INDEX idx ON orders (status, created_at);
+-- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'
+```
+
+**Covering Index:**
+```sql
+CREATE INDEX idx ON users (email) INCLUDE (name, created_at);
+-- Avoids table lookup for SELECT email, name, created_at
+```
+
+**Partial Index:**
+```sql
+CREATE INDEX idx ON users (email) WHERE deleted_at IS NULL;
+-- Smaller index, only includes active users
+```
+
+**RLS Policy (Optimized):**
+```sql
+CREATE POLICY policy ON orders
+  USING ((SELECT auth.uid()) = user_id);  -- Wrap in SELECT!
+```
+
+**UPSERT:**
+```sql
+INSERT INTO settings (user_id, key, value)
+VALUES (123, 'theme', 'dark')
+ON CONFLICT (user_id, key)
+DO UPDATE SET value = EXCLUDED.value;
+```
+
+**Cursor Pagination:**
+```sql
+SELECT * FROM products WHERE id > $last_id ORDER BY id LIMIT 20;
+-- O(1) vs OFFSET which is O(n)
+```
+
+**Queue Processing:**
+```sql
+UPDATE jobs SET status = 'processing'
+WHERE id = (
+  SELECT id FROM jobs WHERE status = 'pending'
+  ORDER BY created_at LIMIT 1
+  FOR UPDATE SKIP LOCKED
+) RETURNING *;
+```
+
+### Anti-Pattern Detection
+
+```sql
+-- Find unindexed foreign keys
+SELECT conrelid::regclass, a.attname
+FROM pg_constraint c
+JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY(c.conkey)
+WHERE c.contype = 'f'
+  AND NOT EXISTS (
+    SELECT 1 FROM pg_index i
+    WHERE i.indrelid = c.conrelid AND a.attnum = ANY(i.indkey)
+  );
+
+-- Find slow queries
+SELECT query, mean_exec_time, calls
+FROM pg_stat_statements
+WHERE mean_exec_time > 100
+ORDER BY mean_exec_time DESC;
+
+-- Check table bloat
+SELECT relname, n_dead_tup, last_vacuum
+FROM pg_stat_user_tables
+WHERE n_dead_tup > 1000
+ORDER BY n_dead_tup DESC;
+```
+
+### Configuration Template
+
+```sql
+-- Connection limits (adjust for RAM)
+ALTER SYSTEM SET max_connections = 100;
+ALTER SYSTEM SET work_mem = '8MB';
+
+-- Timeouts
+ALTER SYSTEM SET idle_in_transaction_session_timeout = '30s';
+ALTER SYSTEM SET statement_timeout = '30s';
+
+-- Monitoring
+CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
+
+-- Security defaults
+REVOKE ALL ON SCHEMA public FROM public;
+
+SELECT pg_reload_conf();
+```
+
+## Related
+
+- Agent: `database-reviewer` - Full database review workflow
+- Skill: `clickhouse-io` - ClickHouse analytics patterns
+- Skill: `backend-patterns` - API and backend patterns
+
+---
+
+*Based on [Supabase Agent Skills](https://github.com/supabase/agent-skills) (MIT License)*