Hold codex in thinking while a turn is running

Codex uses the osc_title_stability idle strategy, but it draws its
progress in the pane body ('Working … esc to interrupt'), not the OSC
title. The title goes stable mid-turn, so ~2s later the classifier
declared codex idle while it was still working. Add a thinking-promoter
pattern ((?i)esc to interrupt) to the codex built-in preset; classify()
checks promoter regexes against the rendered screen before the
title-stability verdict, so codex stays in thinking until the turn's
in-progress footer actually disappears.

Resolves the [CODEX IDLE] TODO item.

internal/preset/preset_test.go

@@ -27,6 +27,13 @@ func TestLoadUsesBuiltInDefaultsWithoutWritingConfig(t *testing.T) {
 	if claude.IdleDetection == nil || len(claude.IdleDetection.PermissionPatterns) == 0 {
 		t.Fatalf("built-in claude missing permission patterns: %+v", claude.IdleDetection)
 	}
+	codex := presetByName(set.Agents, "codex")
+	if codex == nil {
+		t.Fatal("missing built-in codex preset")
+	}
+	if codex.IdleDetection == nil || len(codex.IdleDetection.ThinkingPatterns) == 0 {
+		t.Fatalf("built-in codex missing thinking patterns: %+v", codex.IdleDetection)
+	}
 }
 
 func TestLoadMergesUserOverlayIntoBuiltInPreset(t *testing.T) {