Agent connections were using retryMs=0, causing a tight loop when the
/connection file read failed (e.g., toolsConn nil). The loop was:
connection fails → disconnected signal → refreshSessions →
reconcileAgentConnections → new connection → fails immediately → repeat
Fix by:
- Add maxRetries parameter to NinePConnection::start()
- Use 5s retry interval with max 3 retries for agent connections
- Reset retry count on successful connection
Agent connections no longer retry on their own. When an agent is
killed, the connection goes to Disconnected and stays there.
reconcileAgentConnections (triggered by refreshSessions) is the
sole authority for creating/removing connections.
For the happy path (probe returns normally), the connection
immediately re-probes without a timer delay.