refactor(v2): remove trigger_credential_collection MCP tool

Drops the in-chat credential-collection flow introduced in e92b245. Agents
can no longer collect API keys via a secure modal — users must add secrets
through OneCLI directly. Keeps the OneCLI manual-approval handler and
threaded-routing work from the same commit intact.

Removed:
* container/agent-runner/src/mcp-tools/credentials.ts (MCP tool)
* src/credentials.ts (host-side modal/OneCLI pipeline)
* src/db/credentials.ts + migration 005 (pending_credentials table)
* src/onecli-secrets.ts (createSecret CLI facade, only caller was credentials.ts)
* findCredentialResponse from agent-runner DB layer
* PendingCredential types
* Four credential hooks from ChannelSetup (getCredentialForModal,
  onCredentialReject, onCredentialSubmit, onCredentialChannelUnsupported)
* Credential card/modal handling in chat-sdk-bridge (nccr/nccm prefixes,
  Modal/TextInput imports)
* credential_request text fallback in WhatsApp adapter
* request_credential system-action case in delivery.ts

Added:
* Migration 009 drops pending_credentials on existing installs.

Vercel skill now tells the agent to ask the user to register the token via
OneCLI instead of invoking the removed tool.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/delivery.ts

@@ -865,12 +865,6 @@ async function handleSystemAction(
       break;
     }
 
-    case 'request_credential': {
-      const { handleCredentialRequest } = await import('./credentials.js');
-      await handleCredentialRequest(content, session);
-      break;
-    }
-
     default:
       log.warn('Unknown system action', { action });
   }