agents: explain a pane's screen classification

pane.explain (and `signaltty pane explain`) traces the rules the screen
tick would use for a pane against its current title and screen, read-only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
94df1eff59Lucenx9 committed on 10/9/2026, 10:11:44 AMparent3f63f1b
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