Gmail folder auto-advance + /forward command

- Reply-cycle auto-advance: staff reply files the thread to "Awaiting Reply",
  a customer response files it to "Needs Response" (new GMAIL_LABEL_AWAITING_REPLY
  / GMAIL_LABEL_NEEDS_RESPONSE labels + autoAdvanceFolder, which only moves
  threads still in the auto-cycle and leaves hand-filed folders alone)
- /forward: forward a ticket's email to another address (handlers/commands/forward.js
  + forward composition in services/gmail.js)
- Tests for the auto-advance cycle; label fixtures updated for the new labels

.env.example

@@ -107,6 +107,8 @@ FORCE_CLOSE_TIMER_SECONDS=60              # Seconds to wait before force-closing
 GMAIL_POLL_INTERVAL_SECONDS=30            # Gmail poll interval in seconds (default 30)
 GMAIL_POLL_ENABLED=                       # Inbound email flow master switch; "false" disables polling (default on). Toggle at runtime with /email on|off
 GMAIL_LABEL_TRIAGE=                        # Gmail label for newly created tickets (default "Triage"); auto-created if missing
+GMAIL_LABEL_AWAITING_REPLY=                # Gmail label set when staff reply emails the customer (default "Awaiting Reply")
+GMAIL_LABEL_NEEDS_RESPONSE=                # Gmail label set when the customer responds (default "Needs Response")
 GMAIL_LABEL_ESCALATED=                     # Gmail label for escalated tickets (default "Escalated")
 GMAIL_LABEL_RESOLVED=                      # Gmail label for resolved/closed tickets (default "Resolved")
 GMAIL_LABEL_FOR_JAKE=                      # /folder option (default "For Jake")