add file/image paste to the Claude composer via native clipboard

Flutter's clipboard is text-only and tmux/send-keys carry text only, so
a pasted file or image must reach Claude as an @path reference (per the
T-134 spike). A native clide/clipboard MethodChannel reads the non-text
clipboard: GTK (gtk_clipboard_wait_for_image/uris) on Linux, NSPasteboard
on macOS. The composer overrides PasteTextIntent — Ctrl/Cmd+V resolves a
file path or writes a clipboard image to a cache dir, inserts the @path,
and falls back to plain-text paste otherwise. No new package dependency.

macOS handler is written but unverified on this Linux box — needs a build
on a Mac. Linux path builds and is covered by tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/builtin/claude/claude_composer_test.dart

@@ -79,5 +79,32 @@ void main() {
       await pump(tester, enabled: false);
       expect(tester.widget<EditableText>(find.byType(EditableText)).readOnly, isTrue);
     });
+
+    testWidgets('Ctrl+V routes through the paste resolver and inserts its @path', (tester) async {
+      // WidgetsApp provides DefaultTextEditingShortcuts in the real app;
+      // the bare harness doesn't, so wrap explicitly to map Ctrl+V ->
+      // PasteTextIntent, which the composer's Actions override intercepts.
+      await tester.pumpWidget(harness(
+        f,
+        DefaultTextEditingShortcuts(
+          child: ClaudeComposer(
+            onSubmit: (_) {},
+            pasteResolver: () async => '@/tmp/shot.png',
+          ),
+        ),
+      ));
+      tester.widget<EditableText>(find.byType(EditableText)).focusNode.requestFocus();
+      await tester.pump();
+
+      await tester.sendKeyDownEvent(LogicalKeyboardKey.control);
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyV);
+      await tester.sendKeyUpEvent(LogicalKeyboardKey.control);
+      await tester.pumpAndSettle();
+
+      expect(
+        tester.widget<EditableText>(find.byType(EditableText)).controller.text,
+        '@/tmp/shot.png',
+      );
+    });
   });
 }