API key rotation per provider for DeepSeek Harness

Key features

• Key pools per provider — list the API keys (as credential/env names) that a provider may rotate through.
• Per-provider API key rotation for DeepSeek Harness (dsh).
• The plugin patches ctx.credentials.resolve so a pool reference resolves to the current healthy key (round-robin, skipping keys in cooldown).
• Key values never leave your Credentials store
• The provider you picked stays the provider — rotation swaps the key, never the route, so a multi-call turn does not break.