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/SKILL.md

@@ -0,0 +1,57 @@
+---
+name: supabase-postgres-best-practices
+description: Postgres performance optimization and best practices from Supabase. Use this skill when writing, reviewing, or optimizing Postgres queries, schema designs, or database configurations.
+license: MIT
+metadata:
+  author: supabase
+  version: "1.0.0"
+---
+
+# Supabase Postgres Best Practices
+
+Comprehensive performance optimization guide for Postgres, maintained by Supabase. Contains rules across 8 categories, prioritized by impact to guide automated query optimization and schema design.
+
+## When to Apply
+
+Reference these guidelines when:
+- Writing SQL queries or designing schemas
+- Implementing indexes or query optimization
+- Reviewing database performance issues
+- Configuring connection pooling or scaling
+- Optimizing for Postgres-specific features
+- Working with Row-Level Security (RLS)
+
+## Rule Categories by Priority
+
+| Priority | Category | Impact | Prefix |
+|----------|----------|--------|--------|
+| 1 | Query Performance | CRITICAL | `query-` |
+| 2 | Connection Management | CRITICAL | `conn-` |
+| 3 | Security & RLS | CRITICAL | `security-` |
+| 4 | Schema Design | HIGH | `schema-` |
+| 5 | Concurrency & Locking | MEDIUM-HIGH | `lock-` |
+| 6 | Data Access Patterns | MEDIUM | `data-` |
+| 7 | Monitoring & Diagnostics | LOW-MEDIUM | `monitor-` |
+| 8 | Advanced Features | LOW | `advanced-` |
+
+## How to Use
+
+Read individual rule files for detailed explanations and SQL examples:
+
+```
+rules/query-missing-indexes.md
+rules/schema-partial-indexes.md
+rules/_sections.md
+```
+
+Each rule file contains:
+- Brief explanation of why it matters
+- Incorrect SQL example with explanation
+- Correct SQL example with explanation
+- Optional EXPLAIN output or metrics
+- Additional context and references
+- Supabase-specific notes (when applicable)
+
+## Full Compiled Document
+
+For the complete guide with all rules expanded: `AGENTS.md`