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/Program.cs

@@ -82,6 +82,7 @@ builder.Services.AddSingleton<PlanningMergeOrchestrator>();
 builder.Services.AddSingleton<PlanningChainCoordinator>();
 builder.Services.AddSingleton<LiveSessionRegistry>();
 builder.Services.AddSingleton<InteractiveSessionService>();
+builder.Services.AddHostedService<IdleSessionReaper>();
 
 // Queue dispatch primitives. QueueWaker holds the wake semaphore; the queue picker
 // performs atomic Queued→Running claim. Both injected into the state service so it