fix(sdr-agent): fix agent runtime lifecycle and mcp config

- Replace self._executor with self._agent_runtime (AgentRuntime | None)
- Import AgentRuntime for proper type annotation
- Add missing await self._agent_runtime.start() in start() — runtime
  was created but never started, causing silent failures at runtime
- Add self._agent_runtime = None reset in stop() for clean restart
- Remove redundant self._graph is None guard in trigger_and_wait()
- Update mcp_servers.json with hive-tools server config
- Add credential file patterns to .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -78,3 +78,9 @@ core/tests/*dumps/*
 screenshots/*
 
 .gemini/*
+
+# OAuth credentials (never commit)
+client_secret.json
+token.pickle
+token.json
+get_gmail_token.py