Document intentional error suppressions with comments (gt-zn9m)

All 156 instances of _ = error suppression in non-test code now have
explanatory comments documenting why the error is intentionally ignored.

Categories of intentional suppressions:
- non-fatal: session works without these - tmux environment setup
- non-fatal: theming failure does not affect operation - visual styling
- best-effort cleanup - defer cleanup on failure paths
- best-effort notification - mail/notifications that should not block
- best-effort interrupt - graceful shutdown attempts
- crypto/rand.Read only fails on broken system - random ID generation
- output errors non-actionable - fmt.Fprint to io.Writer

This addresses the silent failure and debugging concerns raised in the
issue by making the intentionality explicit in the code.

Generated with Claude Code https://claude.com/claude-code

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

internal/mail/router.go

@@ -167,7 +167,7 @@ func (r *Router) Send(msg *Message) error {
 		return fmt.Errorf("sending message: %w", err)
 	}
 
-	// Notify recipient if they have an active session
+	// Notify recipient if they have an active session (best-effort notification)
 	// Skip notification for self-mail (handoffs to future-self don't need present-self notified)
 	if !isSelfMail(msg.From, msg.To) {
 		_ = r.notifyRecipient(msg)