Use bytes.NewReader and add Dir/Env/Stdin coverage

Replaces the handwritten bytesBuf/eof workaround with the canonical
bytes.NewReader + io.EOF, and adds behavioural tests for the Dir, Env,
and Stdin branches of Real.Run that the original plan didn't cover.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/runner/runner.go

@@ -1,6 +1,7 @@
 package runner
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os/exec"
@@ -32,7 +33,7 @@ func (Real) Run(ctx context.Context, s Spec) ([]byte, error) {
 		cmd.Env = s.Env
 	}
 	if len(s.Stdin) > 0 {
-		cmd.Stdin = bytesReader(s.Stdin)
+		cmd.Stdin = bytes.NewReader(s.Stdin)
 	}
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -41,25 +42,6 @@ func (Real) Run(ctx context.Context, s Spec) ([]byte, error) {
 	return out, nil
 }
 
-// bytesReader avoids a bytes import in this small file.
-func bytesReader(b []byte) *bytesBuf { return &bytesBuf{b: b} }
-
-type bytesBuf struct {
-	b []byte
-	i int
-}
-
-func (r *bytesBuf) Read(p []byte) (int, error) {
-	if r.i >= len(r.b) {
-		return 0, eof
-	}
-	n := copy(p, r.b[r.i:])
-	r.i += n
-	return n, nil
-}
-
-var eof = fmt.Errorf("EOF")
-
 // Call is a recorded invocation made against a Fake.
 type Call struct {
 	Name string

internal/runner/runner_test.go

@@ -58,3 +58,49 @@ func TestFakeRunner_UnconfiguredCallFailsLoudly(t *testing.T) {
 		t.Fatal("expected error for unconfigured call")
 	}
 }
+
+func TestRealRunner_StdinIsPipedToCommand(t *testing.T) {
+	r := runner.Real{}
+	out, err := r.Run(context.Background(), runner.Spec{
+		Name:  "cat",
+		Stdin: []byte("piped-payload"),
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(string(out), "piped-payload") {
+		t.Fatalf("output %q does not contain piped payload", out)
+	}
+}
+
+func TestRealRunner_DirSetsWorkingDirectory(t *testing.T) {
+	dir := t.TempDir()
+	r := runner.Real{}
+	out, err := r.Run(context.Background(), runner.Spec{
+		Name: "pwd",
+		Dir:  dir,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// macOS resolves /var/folders/... → /private/var/folders/... so just
+	// match the trailing tempdir name.
+	if !strings.Contains(string(out), dir[strings.LastIndex(dir, "/"):]) {
+		t.Fatalf("pwd output %q does not contain %q", out, dir)
+	}
+}
+
+func TestRealRunner_EnvReplacesProcessEnv(t *testing.T) {
+	r := runner.Real{}
+	out, err := r.Run(context.Background(), runner.Spec{
+		Name: "sh",
+		Args: []string{"-c", "echo $WAILS_TEST"},
+		Env:  []string{"WAILS_TEST=hello-from-spec-env"},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(string(out), "hello-from-spec-env") {
+		t.Fatalf("output %q does not contain expected env value", out)
+	}
+}