register live GUI panes into clide pane list (T-219)

Per D-6 parity / D-83, make the kernel tabs the user sees (Claude,
Files, Editor, viewers) visible to the CLI. The PTY-backed PaneRegistry
can't model widget panes, so rather than mirror state (and risk drift),
pane.list snapshots the kernel PanelRegistry + LayoutArrangement at
request time via an injected view-pane source.

New Flutter-free ViewPane value type + snapshotViewPanes kernel bridge;
the daemonClientFactory now passes the PanelRegistry through so the
dispatcher can read it. pane.list merges PTY panes (source absent) with
UI tabs (source: ui, with slot/title/active/visible).

Acceptance (GUI-open: pane list enumerates live panes) needs a running
app to confirm end-to-end; unit-tested at the snapshot + dispatch level.

Closes T-219 (under T-218 / T-208 'Give Claude hands').

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

lib/src/panes/view_pane.dart

@@ -0,0 +1,48 @@
+/// A non-PTY pane the user sees in the GUI — a kernel tab (Claude, Files,
+/// Editor, a viewer) surfaced so `pane list` reflects the live workspace,
+/// not only PTY-spawned panes (T-219, D-6 parity / D-83).
+///
+/// The PTY-backed [Pane] can't represent these: a tab has no child process.
+/// Rather than mirror state into [PaneRegistry] (and risk it drifting from
+/// the live UI), the kernel snapshots its `PanelRegistry` into these value
+/// objects at request time — `pane.list` reads them through an injected
+/// source. Pure data, Flutter-free, so it serialises on the IPC wire and
+/// stays usable from `dart test`.
+library;
+
+class ViewPane {
+  const ViewPane({
+    required this.id,
+    required this.slot,
+    required this.title,
+    required this.active,
+    required this.visible,
+  });
+
+  /// Stable contribution id (e.g. `claude`, `files`, `editor`) — the same id
+  /// `pane.focus` would target.
+  final String id;
+
+  /// The slot the tab lives in (`sidebar` / `workspace` / `context`).
+  final String slot;
+
+  /// Display title the user sees on the tab.
+  final String title;
+
+  /// Whether this is the active (front) tab in its slot — the focus state
+  /// the acceptance asks for.
+  final bool active;
+
+  /// Whether the tab's slot is currently visible (not collapsed/hidden).
+  final bool visible;
+
+  Map<String, Object?> toJson() => {
+        'id': id,
+        'kind': 'view',
+        'slot': slot,
+        'title': title,
+        'active': active,
+        'visible': visible,
+        'source': 'ui',
+      };
+}