Keep agent completions unread until their pane is viewed - #781

Summary

Keep completed panes unread until they are viewed in the active, visible Prowl window. Use the shared viewing predicate for polling and automatic focus acknowledgement, including a fresh visibility check after asynchronous session resolution.

Preserve acknowledgements made while session resolution is suspended, including their timestamp. Discard the pending observation if other pane state changed during suspension. A later unviewed working/blocked-to-idle transition still becomes Done.

Fixes #777.

Validation

• Reproduced the stale acknowledgement on the previous head with a controlled resolver suspension: Done reappeared and the acknowledgement timestamp was overwritten.
• The regression passes with the fix, with and without new session metadata.
• Related local Xcode tests passed in two runs (62 and 26 test methods), including polling, focus, session retention, Active Agents, Island, entry coalescing/deduplication, and state changes during suspension.
• make check passed, including 146 script tests.
• make build-app passed.
• git diff --check passed.
• No live-agent GUI reproduction was performed for this state-management fix.

Related work and credit

Thanks to @SunChJ for the original implementation in #778, resubmitted as #782, and to @onevtail for identifying the session-resolution race in that review. This PR consolidates the shared fix with behavior-level regression coverage and resolves that race. #783 is the separate Canvas viewing companion.