dock Claude's task list above the composer (T-308)

Claude's TodoWrite checklist was invisible. Add a TaskItem/TaskStatus
model + a latest-wins parser (taskListFrom) that reads the most recent
TodoWrite tool call (it replaces the whole list each time), and a compact
display-only ClaudeTaskDock pinned between the conversation and the
composer: collapsed to "N tasks · M done" + the current in-progress item,
expandable to the full checklist with per-item status glyphs + a11y
labels. Hidden when there are no tasks. Parser + widget tests.

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

lib/builtin/claude/src/claude_pane.dart

@@ -9,6 +9,7 @@ import 'claude_banner.dart';
 import 'claude_composer.dart';
 import 'claude_config.dart';
 import 'claude_status.dart';
+import 'claude_task_dock.dart';
 import 'clipboard_paste.dart';
 import 'activity_cluster.dart' show foldLevelFromName, kActivityFoldLevelKey;
 import 'conversation_controller.dart';
@@ -21,6 +22,7 @@ import 'session_orchestrator.dart';
 import 'session_picker.dart';
 import 'slash_commands.dart';
 import 'stream_json_session.dart';
+import 'task_list.dart';
 import 'transcript_reader.dart';
 
 /// The Claude conversation pane. Drives `claude` over the stream-json control
@@ -513,6 +515,12 @@ class _ClaudePaneState extends State<ClaudePane> {
                   ),
                 ),
               ),
+              // Claude's task list, docked above the composer (T-308). Rebuilds
+              // with the conversation; renders nothing when there are no tasks.
+              ListenableBuilder(
+                listenable: _conversation!,
+                builder: (_, __) => ClaudeTaskDock(tasks: taskListFrom(_conversation!.items)),
+              ),
               // An open prompt takes the composer's space and hides the text
               // input until it's answered, so interaction stays out of the
               // conversation stream (D-78).