feat(tasks): add inherited context and resilient LLM execution

Add read-only task context inheritance, ordered task-level LLM failover, live Worker controls, cascading task cleanup, FIFO task concurrency, cancellation diagnostics, UI updates, tests, build script, and changelog.

cmd/artex/main.go

@@ -73,14 +73,8 @@ func main() {
 
 	sigCtx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
-	// re-wrap with a named cause so a run cancelled by shutdown says "后端进程正在
-	// 关停" in its trace instead of a bare context.Canceled (see agent/cancelcause.go).
-	ctx, shutdown := context.WithCancelCause(sigCtx)
+	ctx, shutdown := shutdownContext(sigCtx)
 	defer shutdown(agent.AbortShutdown)
-	go func() {
-		<-sigCtx.Done()
-		shutdown(agent.AbortShutdown)
-	}()
 
 	mgr, err := server.NewManager(*dataDir, *proxy)
 	if err != nil {
@@ -113,3 +107,18 @@ func main() {
 	defer cancel()
 	_ = httpSrv.Shutdown(shutdownCtx)
 }
+
+// shutdownContext deliberately does not derive from signalCtx. If it did, the
+// parent's plain context.Canceled could win the race before AbortShutdown was
+// attached to the child, losing the diagnostic cause in every running Agent.
+func shutdownContext(signalCtx context.Context) (context.Context, context.CancelCauseFunc) {
+	ctx, shutdown := context.WithCancelCause(context.Background())
+	go func() {
+		select {
+		case <-signalCtx.Done():
+			shutdown(agent.AbortShutdown)
+		case <-ctx.Done():
+		}
+	}()
+	return ctx, shutdown
+}

cmd/artex/main_test.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"context"
+	"testing"
+
+	"github.com/Autumn-27/artex/agent"
+)
+
+func TestShutdownContextPreservesNamedCause(t *testing.T) {
+	signalCtx, signalCancel := context.WithCancel(context.Background())
+	ctx, shutdown := shutdownContext(signalCtx)
+	defer shutdown(nil)
+	signalCancel()
+	<-ctx.Done()
+	if code, _, _, ok := agent.AbortReason(ctx); !ok || code != "shutdown" {
+		t.Fatalf("code=%q ok=%v, want shutdown", code, ok)
+	}
+}