[DISCORDSH] Implement vault secret retrieval in discordsh bot

[h0lybyte]

Parent

Part of #7849

Description

Implement runtime secret retrieval from Supabase Vault in the discordsh bot. The bot should fetch GitHub PATs and other credentials from the vault instead of relying on hardcoded env vars.

Acceptance Criteria

• Bot connects to Supabase Vault on startup
• GitHub PAT retrieved securely at runtime
• Fallback/error handling for missing or expired secrets