add clide status orientation snapshot; close Epic C (T-221)

clide status returns a one-shot snapshot for an orienting agent: the
workspace root, a git summary (branch/ahead/behind/clean/counts), the
active editor buffer + selection, the read-only readers' viewed docs
(new ReaderNavRegistry.currentByReader, the T-220 fold), focusedFile,
the live view-pane list (T-219), and the layout (slots + visibility +
focus mode). Previously 'status' was an unknown command (exit 3).

The verb handler (status_command.dart) is a thin Flutter-free wrapper;
the snapshot is assembled in main.dart where the live kernel + subsystem
state is in scope, with readerNav captured post-boot. Composite shape is
verified live; the pieces are unit-tested.

Closes T-221 and T-218 (Epic C) under T-208 'Give Claude hands' --
the observe half of D-6 parity is now in place.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/kernel/reader_nav_test.dart

@@ -143,4 +143,14 @@ void main() {
     final c = reg.navFor('builtin.decisions', dataKey: 'id');
     expect(identical(a, c), isFalse);
   });
+
+  test('currentByReader maps each reader to its viewed doc; omits empty (T-221)', () {
+    final reg = ReaderNavRegistry(bus);
+    addTearDown(reg.dispose);
+    final md = reg.navFor('builtin.markdown', dataKey: 'path');
+    reg.navFor('builtin.decisions', dataKey: 'id'); // created but nothing viewed
+    expect(reg.currentByReader, isEmpty);
+    md.open('docs/initial-plan.md');
+    expect(reg.currentByReader, {'builtin.markdown': 'docs/initial-plan.md'});
+  });
 }