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/conn-idle-timeout.md

@@ -0,0 +1,46 @@
+---
+title: Configure Idle Connection Timeouts
+impact: HIGH
+impactDescription: Reclaim 30-50% of connection slots from idle clients
+tags: connections, timeout, idle, resource-management
+---
+
+## Configure Idle Connection Timeouts
+
+Idle connections waste resources. Configure timeouts to automatically reclaim them.
+
+**Incorrect (connections held indefinitely):**
+
+```sql
+-- No timeout configured
+show idle_in_transaction_session_timeout;  -- 0 (disabled)
+
+-- Connections stay open forever, even when idle
+select pid, state, state_change, query
+from pg_stat_activity
+where state = 'idle in transaction';
+-- Shows transactions idle for hours, holding locks
+```
+
+**Correct (automatic cleanup of idle connections):**
+
+```sql
+-- Terminate connections idle in transaction after 30 seconds
+alter system set idle_in_transaction_session_timeout = '30s';
+
+-- Terminate completely idle connections after 10 minutes
+alter system set idle_session_timeout = '10min';
+
+-- Reload configuration
+select pg_reload_conf();
+```
+
+For pooled connections, configure at the pooler level:
+
+```ini
+# pgbouncer.ini
+server_idle_timeout = 60
+client_idle_timeout = 300
+```
+
+Reference: [Connection Timeouts](https://www.postgresql.org/docs/current/runtime-config-client.html#GUC-IDLE-IN-TRANSACTION-SESSION-TIMEOUT)