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/types.go

@@ -140,14 +140,14 @@ func NewReplyMessage(from, to, subject, body string, original *Message) *Message
 // generateID creates a random message ID.
 func generateID() string {
 	b := make([]byte, 8)
-	_, _ = rand.Read(b)
+	_, _ = rand.Read(b) // crypto/rand.Read only fails on broken system
 	return "msg-" + hex.EncodeToString(b)
 }
 
 // generateThreadID creates a random thread ID.
 func generateThreadID() string {
 	b := make([]byte, 6)
-	_, _ = rand.Read(b)
+	_, _ = rand.Read(b) // crypto/rand.Read only fails on broken system
 	return "thread-" + hex.EncodeToString(b)
 }