feat(skills): add supabase postgres best practices skill and update the Official Sources to include supabase/agent-skills repo

- Adds `skills/postgres-best-practices/`: A new skill containing comprehensive Postgres performance optimization rules and guidelines from Supabase.
- The skill includes rules for query performance, connection management, security (RLS), and schema design.
- Updates `README.md` to include the new skill in the directory.

skills/postgres-best-practices/rules/_sections.md

@@ -0,0 +1,39 @@
+# Section Definitions
+
+This file defines the rule categories for Postgres best practices. Rules are automatically assigned to sections based on their filename prefix.
+
+Take the examples below as pure demonstrative. Replace each section with the actual rule categories for Postgres best practices.
+
+---
+
+## 1. Query Performance (query)
+**Impact:** CRITICAL
+**Description:** Slow queries, missing indexes, inefficient query plans. The most common source of Postgres performance issues.
+
+## 2. Connection Management (conn)
+**Impact:** CRITICAL
+**Description:** Connection pooling, limits, and serverless strategies. Critical for applications with high concurrency or serverless deployments.
+
+## 3. Security & RLS (security)
+**Impact:** CRITICAL
+**Description:** Row-Level Security policies, privilege management, and authentication patterns.
+
+## 4. Schema Design (schema)
+**Impact:** HIGH
+**Description:** Table design, index strategies, partitioning, and data type selection. Foundation for long-term performance.
+
+## 5. Concurrency & Locking (lock)
+**Impact:** MEDIUM-HIGH
+**Description:** Transaction management, isolation levels, deadlock prevention, and lock contention patterns.
+
+## 6. Data Access Patterns (data)
+**Impact:** MEDIUM
+**Description:** N+1 query elimination, batch operations, cursor-based pagination, and efficient data fetching.
+
+## 7. Monitoring & Diagnostics (monitor)
+**Impact:** LOW-MEDIUM
+**Description:** Using pg_stat_statements, EXPLAIN ANALYZE, metrics collection, and performance diagnostics.
+
+## 8. Advanced Features (advanced)
+**Impact:** LOW
+**Description:** Full-text search, JSONB optimization, PostGIS, extensions, and advanced Postgres features.