feat(netshell): retain approval across unexpected disconnects

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Summary

• require human approval before the first model-initiated SSH connection
• clear server authorization only when the user manually disconnects
• retain authorization across SSH exits, network failures, and connection-wait timeouts so the next model call can reconnect without another prompt
• fail closed when the confirmation service is unavailable
• cover first connection, concurrent calls, manual disconnect, unexpected exit, and no-provider cases

Verification

• pnpm test
• git diff --check