Serve a tools bundle as soon as it is in place, changed or gone, with no restart; say why a bundle cannot start (issue 1945)

node-tools loaded every bundle once at start, so a bundle delivered after
it started (mesh-files on shanks, 2026-10-25 12:08) went unserved until a
hand restart, and a missing file was said as "not executable". The
runtime now watches the entrypoints' directories with inotify, loads a
module again once a burst of changes settles, and discovery answers with
what is served now. A bundle that cannot start says it does not exist, is
not executable with its mode, or the exec error.

Tests: 1945/AC19-AC22 in internal/runtime/reload_test.go.

node-tools/internal/runtime/runtime_test.go

@@ -209,7 +209,7 @@ func TestAnEntrypointThatIsNotExecutableIsRefused(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer stop()
-	if !logs.Has("plain's bundle", "many-alpha.mjs failed to load:", "is not executable; a bundle the runtime serves is started, never imported") {
+	if !logs.Has("plain's bundle", "many-alpha.mjs failed to load:", "is not executable (mode -", "); a bundle the runtime serves is started, never imported") {
 		t.Errorf("the non-executable entrypoint was not refused by name:\n%s", logs.All())
 	}
 	got, err := call(t, asker, "alpha.one@anchor", map[string]any{})