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/_template.md

@@ -0,0 +1,34 @@
+---
+title: Clear, Action-Oriented Title (e.g., "Use Partial Indexes for Filtered Queries")
+impact: MEDIUM
+impactDescription: 5-20x query speedup for filtered queries
+tags: indexes, query-optimization, performance
+---
+
+## [Rule Title]
+
+[1-2 sentence explanation of the problem and why it matters. Focus on performance impact.]
+
+**Incorrect (describe the problem):**
+
+```sql
+-- Comment explaining what makes this slow/problematic
+CREATE INDEX users_email_idx ON users(email);
+
+SELECT * FROM users WHERE email = 'user@example.com' AND deleted_at IS NULL;
+-- This scans deleted records unnecessarily
+```
+
+**Correct (describe the solution):**
+
+```sql
+-- Comment explaining why this is better
+CREATE INDEX users_active_email_idx ON users(email) WHERE deleted_at IS NULL;
+
+SELECT * FROM users WHERE email = 'user@example.com' AND deleted_at IS NULL;
+-- Only indexes active users, 10x smaller index, faster queries
+```
+
+[Optional: Additional context, edge cases, or trade-offs]
+
+Reference: [Postgres Docs](https://www.postgresql.org/docs/current/)