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>

lib/kernel/src/reader_nav.dart

@@ -127,6 +127,15 @@ class ReaderNavRegistry {
   final MessageBus _messages;
   final Map<String, ReaderNav> _navs = {};
 
+  /// The currently-viewed doc for every reader that has one, keyed by
+  /// publisher id (e.g. `builtin.markdown`, `builtin.decisions`). Used by
+  /// `clide status` to surface what the user is reading (T-221, D-6 parity) —
+  /// viewer files aren't editor buffers, so they don't live in EditorRegistry.
+  Map<String, String> get currentByReader => {
+        for (final e in _navs.entries)
+          if (e.value.current != null) e.key: e.value.current!,
+      };
+
   /// The retained [ReaderNav] for [publisherId], created on first use.
   /// [dataKey] is the bus-payload key for this reader's entry.
   ReaderNav navFor(String publisherId, {required String dataKey}) {