plumb: open a plumbed directory as a folder in Kate

When a plumbed path resolves to an existing directory, onPlumbEdit now opens
it as a folder in Kate (acme-style directory open) instead of trying to open
it as a document. Kate accepts a directory on its command line and is
single-instance, so relaunching the hosting kate binary (QCoreApplication::
applicationFilePath, QStandardPaths fallback) on the dir opens it in the
running window. No in-process public API exists to open a folder as a project
(our project bridge is read-only), so this uses the documented command-line
path rather than a guessed slot. 18/18 ctest.

docs/PLAN.md

@@ -346,7 +346,11 @@ Kate flow through the real plumber rules.
   selection, else `Document::wordAt(cursorPosition)`, with the active doc's dir
   as `wdir`; on send failure it falls back to an internal resolver (URL →
   `QDesktopServices`, path[:line] → open here). Incoming `edit` messages are
-  opened via `MainWindow::openUrl` + `activateView` + `setCursorPosition`.
+  opened via `MainWindow::openUrl` + `activateView` + `setCursorPosition`; a
+  plumbed path that resolves to a **directory** opens as a folder in Kate
+  instead (relaunches the hosting kate binary on the dir — Kate is
+  single-instance, so it opens in the running window; no in-process folder-open
+  API exists, so this is the documented command-line path).
 - **Verification**: `test_plumbmsg` + `test_plumbresolve` run headless;
   `test_plumb_live` does a **real round-trip** against a running plumber (send
   `<file>:2` → rules → edit port → `edit()` signal) and **skips+passes** when no

src/plugin/ollieplugin.cpp

@@ -29,6 +29,7 @@
 
 #include <QAction>
 #include <QApplication>
+#include <QCoreApplication>
 #include <QCursor>
 #include <QDateTime>
 #include <QDesktopServices>
@@ -40,6 +41,7 @@
 #include <QKeyEvent>
 #include <QKeySequence>
 #include <QMouseEvent>
+#include <QProcess>
 #include <QRegularExpression>
 #include <QSet>
 #include <QStandardPaths>
@@ -895,6 +897,23 @@ void OllieView::onPlumbEdit(const QString &file, const QString &addr, const QStr
         fi = QFileInfo(QDir(wdir).absoluteFilePath(path));
     }
 
+    // A plumbed directory opens as a folder in Kate (like acme opening a
+    // directory listing). Kate accepts a directory URL on its command line and,
+    // being single-instance, opens it in the running window. We relaunch the
+    // very binary hosting us so it is unambiguously "this Kate". There is no
+    // in-process public API to open a folder as a project, so this is the
+    // documented path, not a guessed one.
+    if (fi.exists() && fi.isDir()) {
+        QString kate = QCoreApplication::applicationFilePath();
+        if (kate.isEmpty() || !QFileInfo(kate).fileName().contains(QLatin1String("kate"))) {
+            kate = QStandardPaths::findExecutable(QStringLiteral("kate"));
+        }
+        if (!kate.isEmpty()) {
+            QProcess::startDetached(kate, {fi.absoluteFilePath()});
+        }
+        return;
+    }
+
     const QUrl url = QUrl::fromLocalFile(fi.absoluteFilePath());
     KTextEditor::View *view = m_mainWindow->openUrl(url);
     if (!view) {