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/metadata.json

@@ -0,0 +1,13 @@
+{
+	"version": "1.0.0",
+	"organization": "Supabase",
+	"date": "January 2026",
+	"abstract": "Comprehensive Postgres performance optimization guide for developers using Supabase and Postgres. Contains performance rules across 8 categories, prioritized by impact from critical (query performance, connection management) to incremental (advanced features). Each rule includes detailed explanations, incorrect vs. correct SQL examples, query plan analysis, and specific performance metrics to guide automated optimization and code generation.",
+	"references": [
+		"https://www.postgresql.org/docs/current/",
+		"https://supabase.com/docs",
+		"https://wiki.postgresql.org/wiki/Performance_Optimization",
+		"https://supabase.com/docs/guides/database/overview",
+		"https://supabase.com/docs/guides/auth/row-level-security"
+	]
+}