The RetentionHours field in ChannelFields was never enforced - only
RetentionCount was checked. Now both EnforceChannelRetention and
PruneAllChannels delete messages older than the configured hours.
Also fixes sling tests that were missing TMUX_PANE and GT_TEST_NO_NUDGE
guards, causing them to inject prompts into active tmux sessions during
test runs.
Fixes: gt-uvnfug
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>