fix(client): resolve default branch from the remote advertisement

A cached bare clone's HEAD points at refs/heads/<branch>, which is written
once at clone time and never updated again — fetch only touches
refs/remotes/*. Syncs without --ref therefore resolved the frozen
clone-time revision forever: the literal "cached version of the repo"
symptom.

Resolve the default branch via remote.List instead: the advertised HEAD
(plain hash or symref target) is the only source of truth. Verified
against the production Forgejo instance; e2e test fails on the pre-fix
binary and passes now.

internal/e2e/sync_default_branch_test.go

@@ -0,0 +1,42 @@
+package e2e
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestSyncDefaultBranchStaysFresh pins the fix for the stale-cache bug: with
+// no ref configured, sync must resolve the default branch from the remote
+// advertisement, never from the cached bare clone's frozen refs/heads ref.
+func TestSyncDefaultBranchStaysFresh(t *testing.T) {
+	h := newHarness(t)
+	env := newSyncEnv(t, h)
+	buildWorkRepo(t, h, env.workRepo)
+
+	// config without "ref": the default-branch resolution path
+	env.config = filepath.Join(env.configDir, "config.json")
+	os.WriteFile(env.config, []byte(`{"repo_url": "`+env.workRepo+`"}`), 0o600)
+
+	if _, err := env.run(t); err != nil {
+		t.Fatal(err)
+	}
+
+	// upstream change on the default branch
+	p := filepath.Join(env.workRepo, "commands/deploy.md")
+	os.WriteFile(p, []byte("---\nname: Deploy\ndescription: updated\n---\nfresh body\n"), 0o644)
+	h.git(t, env.workRepo, "add", "-A")
+	h.git(t, env.workRepo, "commit", "-m", "update command on default branch")
+
+	out, err := env.run(t)
+	if err != nil {
+		t.Fatalf("second sync (default branch) failed: %v\n%s", err, out)
+	}
+	if !contains(out, "updated:  1") {
+		t.Errorf("default-branch sync must pick up the upstream change:\n%s", out)
+	}
+	deployed := readFile(t, filepath.Join(env.home, ".config/opencode/commands/deploy.md"))
+	if !contains(deployed, "fresh body") {
+		t.Error("default-branch sync must deploy the fresh content, not the clone-time revision")
+	}
+}