feat(crash): improve crash logging and mass death detection

Add comprehensive crash logging improvements to help diagnose mass session death events:

- Add TypeSessionDeath and TypeMassDeath event types for feed visibility
- Log pre-death events before killing sessions (who killed, why)
- Add mass death detection in daemon (3+ deaths in 30s triggers alert)
- Add macOS crash report check in gt doctor
- Support session death events in townlog and feed curator

Closes hq-kt1o6

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/session/town.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/steveyegge/gastown/internal/boot"
+	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/tmux"
 )
 
@@ -44,6 +45,14 @@ func StopTownSession(t *tmux.Tmux, ts TownSession, force bool) (bool, error) {
 		time.Sleep(100 * time.Millisecond)
 	}
 
+	// Log pre-death event for crash investigation (before killing)
+	reason := "user shutdown"
+	if force {
+		reason = "forced shutdown"
+	}
+	_ = events.LogFeed(events.TypeSessionDeath, ts.SessionID,
+		events.SessionDeathPayload(ts.SessionID, ts.Name, reason, "gt down"))
+
 	// Kill the session
 	if err := t.KillSession(ts.SessionID); err != nil {
 		return false, fmt.Errorf("killing %s session: %w", ts.Name, err)