provider tests: the hold tests pass on any -count, each starting from a config folder of its own instead of the files the last round left

TestHoldReadsSettingsOnce and TestHoldReadsMigrationsOnce failed on every
-count>=2, without any load ("a visibility before any was set", "moved
before any move"): they wrote settings.json and migrations.json into the
package's one test HOME, so the second round found held-agent's visibility
and held-sub's move already there. The leftover was only those files, not a
package global production relies on: Hold's cache is dropped at the last
release, and filememo re-reads a file that changed. Each of the four hold
tests that writes a magpie file now sets XDG_CONFIG_HOME to a TempDir, as
the rest of the package's tests do.

Before: go test -run TestHoldReads -count=2 fails both. After: -run TestHold
-count=20 -race -shuffle=on passes.

internal/provider/hold_test.go

@@ -46,6 +46,7 @@ func TestHoldBuildsOnce(t *testing.T) {
 // the agents read settings.json for every model of every agent), and a
 // setting saved meanwhile is seen at once.
 func TestHoldReadsSettingsOnce(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir()) // a run starts from no files, not the last run's
 	defer Hold()()
 	if _, ok := VisibleTo("held-agent"); ok {
 		t.Fatal("a visibility before any was set")
@@ -69,6 +70,7 @@ func TestHoldReadsSettingsOnce(t *testing.T) {
 
 // A provider saved while a request holds the catalog is in it at once.
 func TestHoldSeesWrites(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir()) // a run starts from no files, not the last run's
 	if err := Save(Provider{ID: "held", Name: "Held", Chat: "https://held.example/v1", Key: "k", Models: []string{"a"}}); err != nil {
 		t.Fatal(err)
 	}
@@ -100,6 +102,7 @@ func TestHoldSeesWrites(t *testing.T) {
 // account written meanwhile has them built again, and a caller changing
 // its list changes no one else's.
 func TestHoldBuildsProvidersOnce(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir()) // a run starts from no files, not the last run's
 	if err := Save(Provider{ID: "once", Name: "Once", Chat: "https://once.example/v1", Key: "k", Models: []string{"a"}}); err != nil {
 		t.Fatal(err)
 	}
@@ -178,6 +181,7 @@ func TestCursorKeychainReadOnce(t *testing.T) {
 // and a move written meanwhile, even one read from the file as it was
 // before, is seen at once.
 func TestHoldReadsMigrationsOnce(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir()) // a run starts from no files, not the last run's
 	defer Hold()()
 	if Moved("held-sub") {
 		t.Fatal("moved before any move")