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/polecat/pending.go

@@ -141,7 +141,7 @@ func CheckInboxForSpawns(townRoot string) ([]*PendingSpawn, error) {
 		pending = append(pending, ps)
 		existing[msg.ID] = true
 
-		// Mark message as read
+		// Mark message as read (non-fatal: message tracking)
 		_ = mailbox.MarkRead(msg.ID)
 	}