Fix vm run guest repo path and add vm acp bridge

Normalize repo-backed guest checkouts to /root/repo so vm run, attach, and
follow-on guest tooling stop depending on the source repository name.

Add `banger vm acp [--cwd] <vm>` as an SSH stdio bridge to guest `opencode acp`,
defaulting to /root/repo when that checkout exists and falling back to /root.
Update the README and CLI coverage around the fixed guest path and ACP command.

Validation: go test ./internal/cli, go test ./..., make build.

internal/cli/cli_test.go

@@ -233,6 +233,21 @@ func TestVMRunFlagsExist(t *testing.T) {
 	}
 }
 
+func TestVMACPFlagsExist(t *testing.T) {
+	root := NewBangerCommand()
+	vm, _, err := root.Find([]string{"vm"})
+	if err != nil {
+		t.Fatalf("find vm: %v", err)
+	}
+	acp, _, err := vm.Find([]string{"acp"})
+	if err != nil {
+		t.Fatalf("find acp: %v", err)
+	}
+	if acp.Flags().Lookup("cwd") == nil {
+		t.Fatal("missing flag \"cwd\"")
+	}
+}
+
 func TestVMCreateFlagsShowStaticDefaults(t *testing.T) {
 	root := NewBangerCommand()
 	vm, _, err := root.Find([]string{"vm"})
@@ -967,6 +982,98 @@ func TestSSHCommandArgs(t *testing.T) {
 	}
 }
 
+func TestRunVMACPBridgesOverSSH(t *testing.T) {
+	origVMSSH := vmSSHFunc
+	origSSHExec := sshExecFunc
+	t.Cleanup(func() {
+		vmSSHFunc = origVMSSH
+		sshExecFunc = origSSHExec
+	})
+
+	vmSSHFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMSSHResult, error) {
+		if socketPath != "/tmp/bangerd.sock" {
+			t.Fatalf("socketPath = %q, want /tmp/bangerd.sock", socketPath)
+		}
+		if idOrName != "devbox" {
+			t.Fatalf("idOrName = %q, want devbox", idOrName)
+		}
+		return api.VMSSHResult{Name: "devbox", GuestIP: "172.16.0.2"}, nil
+	}
+
+	var gotArgs []string
+	var gotStdin string
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		gotArgs = append([]string(nil), args...)
+		data, err := io.ReadAll(stdin)
+		if err != nil {
+			t.Fatalf("ReadAll(stdin): %v", err)
+		}
+		gotStdin = string(data)
+		return nil
+	}
+
+	if err := runVMACP(
+		context.Background(),
+		"/tmp/bangerd.sock",
+		model.DaemonConfig{SSHKeyPath: "/tmp/id_ed25519"},
+		strings.NewReader("client stream"),
+		&bytes.Buffer{},
+		&bytes.Buffer{},
+		"devbox",
+		"",
+	); err != nil {
+		t.Fatalf("runVMACP: %v", err)
+	}
+
+	if gotStdin != "client stream" {
+		t.Fatalf("stdin = %q, want client stream", gotStdin)
+	}
+	joined := strings.Join(gotArgs, " ")
+	for _, want := range []string{
+		"-T",
+		"-F /dev/null",
+		"-i /tmp/id_ed25519",
+		"-o LogLevel=ERROR",
+		"root@172.16.0.2",
+		"bash -lc",
+	} {
+		if !strings.Contains(joined, want) {
+			t.Fatalf("ssh args = %q, want %q", joined, want)
+		}
+	}
+	remoteCommand := gotArgs[len(gotArgs)-1]
+	if !strings.Contains(remoteCommand, `exec opencode acp --cwd "$DIR"`) {
+		t.Fatalf("remote command = %q, want ACP exec", remoteCommand)
+	}
+	if !strings.Contains(remoteCommand, "REPO_DIR='/root/repo'") {
+		t.Fatalf("remote command = %q, want repo fallback", remoteCommand)
+	}
+}
+
+func TestVMACPRemoteCommandDefaultsToRepoThenRoot(t *testing.T) {
+	got := vmACPRemoteCommand("")
+	for _, want := range []string{
+		"REPO_DIR='/root/repo'",
+		"DEFAULT_DIR='/root'",
+		`if [ -d "$REPO_DIR" ]; then DIR="$REPO_DIR"; else DIR="$DEFAULT_DIR"; fi`,
+		`exec opencode acp --cwd "$DIR"`,
+	} {
+		if !strings.Contains(got, want) {
+			t.Fatalf("vmACPRemoteCommand() = %q, want %q", got, want)
+		}
+	}
+}
+
+func TestVMACPRemoteCommandUsesExplicitCWD(t *testing.T) {
+	got := vmACPRemoteCommand("/workspace/project")
+	if !strings.Contains(got, "DIR='/workspace/project'") {
+		t.Fatalf("vmACPRemoteCommand() = %q, want explicit cwd", got)
+	}
+	if strings.Contains(got, "REPO_DIR=") {
+		t.Fatalf("vmACPRemoteCommand() = %q, want no repo fallback", got)
+	}
+}
+
 func TestValidateSSHPrereqs(t *testing.T) {
 	dir := t.TempDir()
 	keyPath := filepath.Join(dir, "id_ed25519")
@@ -1156,6 +1263,7 @@ func TestRunVMRunCreatesImportsAndAttaches(t *testing.T) {
 		Runtime: model.VMRuntime{
 			State:   model.VMStateRunning,
 			GuestIP: "172.16.0.2",
+			DNSName: "devbox.vm",
 		},
 	}
 	vmCreateBeginFunc = func(context.Context, string, api.VMCreateParams) (api.VMCreateBeginResult, error) {
@@ -1716,6 +1824,12 @@ func TestVMRunCheckoutScriptSkipsRepoGitIdentityWhenIncomplete(t *testing.T) {
 	}
 }
 
+func TestVMRunGuestDirIsFixed(t *testing.T) {
+	if got := vmRunGuestDir(); got != "/root/repo" {
+		t.Fatalf("vmRunGuestDir() = %q, want /root/repo", got)
+	}
+}
+
 func TestNewBangerdCommandRejectsArgs(t *testing.T) {
 	cmd := NewBangerdCommand()
 	cmd.SetArgs([]string{"extra"})
@@ -1951,12 +2065,14 @@ func (c *testVMRunGuestClient) StreamTar(ctx context.Context, sourceDir, remoteC
 
 func (c *testVMRunGuestClient) RunScript(ctx context.Context, script string, logWriter io.Writer) error {
 	c.runScriptCalls++
-	if c.runScriptCalls == 1 {
+	switch c.runScriptCalls {
+	case 1:
 		c.script = script
 		return c.checkoutErr
+	default:
+		c.launchScript = script
+		return c.launchErr
 	}
-	c.launchScript = script
-	return c.launchErr
 }
 
 func (c *testVMRunGuestClient) StreamTarEntries(ctx context.Context, sourceDir string, entries []string, remoteCommand string, logWriter io.Writer) error {