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.go

@@ -31,9 +31,6 @@ const (
 	LabelDelete   = "Delete"
 	LabelSaved    = "Saved"
 	repliedPrefix = "Replied-to/"
-	// LabelSent is Gmail's IMAP name for the Sent mailbox. Phase 5 checks it to
-	// confirm a reply joined the original thread.
-	LabelSent = "[Gmail]/Sent Mail"
 )
 
 // ErrNotConfigured means Gmail credentials are missing, so no mailbox action can
@@ -50,6 +47,9 @@ type Conn interface {
 	StoreFlags(uid uint32, op string, flag string) error
 	SearchGmailID(gmailID string) ([]uint32, error)
 	SearchGmailThreadID(threadID string) ([]uint32, error)
+	// SentMailbox returns the server's \Sent mailbox (Gmail may call it
+	// "[Gmail]/Sent Mail" or "[Google Mail]/Sent Mail").
+	SentMailbox() (string, error)
 	Logout() error
 }
 
@@ -164,12 +164,16 @@ func (c *Client) VerifyThreadUpdated(ctx context.Context, threadID string) (bool
 	}
 	defer func() { _ = conn.Logout() }()
 
-	if _, err := conn.Select(LabelSent); err != nil {
-		return false, fmt.Errorf("select %q: %w", LabelSent, err)
+	sent, err := conn.SentMailbox()
+	if err != nil {
+		return false, fmt.Errorf("find the sent mailbox: %w", err)
+	}
+	if _, err := conn.Select(sent); err != nil {
+		return false, fmt.Errorf("select %q: %w", sent, err)
 	}
 	uids, err := conn.SearchGmailThreadID(threadID)
 	if err != nil {
-		return false, fmt.Errorf("search thread %s in %q: %w", threadID, LabelSent, err)
+		return false, fmt.Errorf("search thread %s in %q: %w", threadID, sent, err)
 	}
 	return len(uids) > 0, nil
 }