"fetch failed" hides the network cause; doctor points at agents directories that Grok Bot 0.71.2 no longer writes #157

On macOS with Grok Bot 0.71.2 and grok-bot-cli 0.12.4 (with #145 applied), gbot thread <agent> --gateway --json failed with only:

{"error":{"code":"render-failed","message":"fetch failed"}}

The real failure was a connect timeout from this machine to api2.cursor.sh during EnsureSandBox (undici UND_ERR_CONNECT_TIMEOUT, all three A records). The desktop app's own connections to the same addresses were stuck in SYN_SENT at the same time; a few minutes later the read worked unchanged.

Two things sent us looking in the wrong place:

1. The error drops err.cause, so nothing shows that it was the network, which host, or which step (EnsureSandBox vs the gateway /api/* call).
2. gbot doctor prints Grok Bot app session: not found and found: (none) with the old agent-data/agents / sand-data/agents candidates. Grok Bot 0.71.2 writes neither gateway-descriptor.json nor an agents directory (its data dir has sand-secrets.json and sand-client-persistence/*.blob), so those lines are expected and say nothing about whether the gateway is reachable.

Suggested change: include the step and the cause code/host in the fetch failed message (e.g. EnsureSandBox: connect timeout to api2.cursor.sh), and have doctor try EnsureSandBox with the active-account credentials and report that result instead of the agents-directory search when the descriptor is absent.