feat(rules): every Maildir folder in the Folder dropdown

The dropdown was built from config, which names one subtree per account
and nothing below it, so it offered five entries and no way to say
Drafts or Sent. A rule wants to target those as often as a whole
account.

NotmuchWorker gains requestFolders/foldersReady, walking the tree from
notmuch_database_get_path() and listing every directory holding cur/.
It belongs there because the database root is notmuch's database.path
and the worker owns the only handle that can answer for it; putting the
root in config would be the second source of truth the design refuses.

From the disk rather than from the index: a folder mbsync created and
nothing has landed in yet is still a folder a rule may target, and a
list derived from indexed message paths would not offer it.

The two tests build their own fixture rather than extending the shared
one, which needs a nested folder and would otherwise move seven count
assertions in unrelated tests. Mutation-checked: flattening the walk to
non-recursive fails the listing test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index b3fbed3..9736ab8 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -151,6 +151,18 @@ public slots:
     /// called when the dialog is opened and never on a timer.
     void requestDatabaseStats(quint64 generation);
 
+    /// Every Maildir folder under the database root, as paths relative to it.
+    ///
+    /// From the DISK, not from the index: a folder mbsync created and nothing
+    /// has landed in yet is still a folder a tagging rule may target, and one
+    /// derived from indexed message paths would not offer it.
+    ///
+    /// Here rather than in MainWindow because the database root is
+    /// notmuch's `database.path` and this class owns the only handle that can
+    /// answer for it. Duplicating the path into config is exactly the second
+    /// source of truth the design refuses.
+    void requestFolders();
+
 signals:
     void threadsReady(const QVector<ThreadSummary> &threads, quint64 generation);
     void queryFinished(int totalThreads, quint64 generation);
@@ -175,6 +187,11 @@ signals:
     /// renders as unknown rather than as zero.
     void databaseStatsReady(const DatabaseStats &stats, quint64 generation);
 
+    /// Maildir folders relative to the database root, sorted. No generation:
+    /// the tree on disk does not change under a query, and the one consumer
+    /// asks once when its dialog opens.
+    void foldersReady(const QStringList &folders);
+
     void errorOccurred(const QString &message);
 
 private: