fix(channel): enforce RetentionHours in channel message retention

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>

internal/cmd/sling_test.go

@@ -616,6 +616,7 @@ exit 0
 	t.Setenv(EnvGTRole, "crew")
 	t.Setenv("GT_CREW", "jv")
 	t.Setenv("GT_POLECAT", "")
+	t.Setenv("TMUX_PANE", "") // Prevent inheriting real tmux pane from test runner
 
 	cwd, err := os.Getwd()
 	if err != nil {
@@ -637,6 +638,9 @@ exit 0
 	slingDryRun = true
 	slingNoConvoy = true
 
+	// Prevent real tmux nudge from firing during tests (causes agent self-interruption)
+	t.Setenv("GT_TEST_NO_NUDGE", "1")
+
 	// EXPECTED: gt sling should use daemon mode and succeed
 	// ACTUAL: verifyBeadExists uses --no-daemon and fails with sync error
 	beadID := "jv-v599"
@@ -792,6 +796,7 @@ exit 0
 	t.Setenv(EnvGTRole, "mayor")
 	t.Setenv("GT_POLECAT", "")
 	t.Setenv("GT_CREW", "")
+	t.Setenv("TMUX_PANE", "") // Prevent inheriting real tmux pane from test runner
 
 	cwd, err := os.Getwd()
 	if err != nil {
@@ -819,6 +824,9 @@ exit 0
 	slingVars = nil
 	slingOnTarget = "gt-abc123" // The bug bead we're applying formula to
 
+	// Prevent real tmux nudge from firing during tests (causes agent self-interruption)
+	t.Setenv("GT_TEST_NO_NUDGE", "1")
+
 	if err := runSling(nil, []string{"mol-polecat-work"}); err != nil {
 		t.Fatalf("runSling: %v", err)
 	}