Phase 5 follow-up: discover the \Sent mailbox; record the approved real send (AUD-71)

The approved real test reply was sent and the thread updated (same
X-GM-THRID as the original). The live test exposed one bug: the thread
check hardcoded Gmail's sent mailbox as "[Gmail]/Sent Mail", but this
account exposes it as "[Google Mail]/Sent Mail".

- internal/mailfetch: LIST parser, ListMailboxes, and SentMailbox that
  finds the mailbox flagged \Sent (RFC 6154) with name fallbacks.
- internal/mailops: VerifyThreadUpdated selects the discovered \Sent
  mailbox instead of a hardcoded name.
- cmd/email-agent: -check-thread <id> exercises the check against real
  Gmail without sending mail.
- Tests for LIST parsing, sent-mailbox discovery, and the discovery
  error path.
- docs/phase5-replies.md: record send-as verification, the sent reply's
  thread id, the app confirmation (sent_reply_in_thread=true), and the
  fix.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/mailops/mailops_test.go

@@ -21,6 +21,8 @@ type fakeConn struct {
 	searched    []string
 	searchUIDs  []uint32
 	searchErr   error
+	sentMailbox string
+	sentErr     error
 	loggedOut   bool
 }
 
@@ -49,6 +51,15 @@ func (f *fakeConn) SearchGmailThreadID(id string) ([]uint32, error) {
 	f.searched = append(f.searched, "thrid:"+id)
 	return f.searchUIDs, f.searchErr
 }
+func (f *fakeConn) SentMailbox() (string, error) {
+	if f.sentErr != nil {
+		return "", f.sentErr
+	}
+	if f.sentMailbox == "" {
+		return "[Google Mail]/Sent Mail", nil
+	}
+	return f.sentMailbox, nil
+}
 func (f *fakeConn) Logout() error { f.loggedOut = true; return nil }
 
 func configured() *config.Config {