fix(worker): reap idle interactive sessions so they don't pile up

Interactive/streaming sessions are persistent claude.exe processes that
wait on stdin and never exit on their own. The only teardown was an
explicit StopInteractiveSession from the UI — there is no client-disconnect
or shutdown sweep — so an abandoned chat (UI closed, navigated away,
crashed) kept its claude.exe (+ conhost) alive for the worker's whole
lifetime. Under a long-running autostart worker these accumulate to dozens
of orphaned child processes.

LiveSessionRegistry now tracks per-session activity (Touch on every output
line and user action) and exposes ReapIdleAsync, which stops sessions idle
past a timeout while skipping any with a turn in flight. IdleSessionReaper
(BackgroundService) sweeps every 5 min; idle timeout defaults to 30 min,
configurable via interactive_idle_timeout_minutes (0 disables).

src/ClaudeDo.Worker/Planning/InteractiveSessionService.cs

@@ -70,7 +70,11 @@ public sealed class InteractiveSessionService
             "--permission-mode", "auto",
         };
 
-        Func<string, Task> onLine = line => _broadcaster.TaskMessage(taskId, "[stdout] " + line);
+        Func<string, Task> onLine = line =>
+        {
+            _registry.Touch(taskId);
+            return _broadcaster.TaskMessage(taskId, "[stdout] " + line);
+        };
 
         ILiveSession session;
         Task exitTask;
@@ -124,19 +128,26 @@ public sealed class InteractiveSessionService
     {
         if (!_registry.TryGet(taskId, out var session))
             throw new InvalidOperationException("No interactive session is running for this task.");
+        _registry.Touch(taskId);
         await session.SendUserMessageAsync(text, ct);
     }
 
     public async Task RemoveQueuedAsync(string taskId, string text, CancellationToken ct)
     {
         if (_registry.TryGet(taskId, out var session))
+        {
+            _registry.Touch(taskId);
             await session.RemoveQueuedAsync(text, ct);
+        }
     }
 
     public async Task InterruptAsync(string taskId, CancellationToken ct)
     {
         if (_registry.TryGet(taskId, out var session))
+        {
+            _registry.Touch(taskId);
             await session.InterruptAsync(ct);
+        }
     }
 
     public async Task StopAsync(string taskId, CancellationToken ct)