Read channel connection progress

Use attemptId returned by create_channel_connection_link. Read saved progress on user request; do not run unbounded polling. Pending/authorizing/selecting requires human browser completion. Processing is uncertain and must not trigger another callback. Completed/partial returns actual saved channelIds; inspect each with get_channel to verify readiness. Failed or expired does not prove no channel was saved: reconcile returned IDs and list_channels before a new attempt. Records expire after 24 hours; a later 404 is a retention gap, not evidence of no connection. Requires channels:read.