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.

db/task_context_unit_test.go

@@ -0,0 +1,27 @@
+package db
+
+import (
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+func TestTruncateUTF8PreservesValidEncoding(t *testing.T) {
+	t.Parallel()
+	input := strings.Repeat("额度不足", 400)
+	got := truncateUTF8(input, 1000)
+	if len(got) > 1000 {
+		t.Fatalf("truncated value has %d bytes, want at most 1000", len(got))
+	}
+	if !utf8.ValidString(got) {
+		t.Fatalf("truncated value is not valid UTF-8: %q", got[len(got)-8:])
+	}
+}
+
+func TestTruncateUTF8RepairsInvalidInput(t *testing.T) {
+	t.Parallel()
+	got := truncateUTF8("bad\xffvalue", 1000)
+	if !utf8.ValidString(got) {
+		t.Fatalf("repaired value is not valid UTF-8: %q", got)
+	}
+}