feat(open): output instance names to stdout when piped

Enable chaining with brev open by outputting instance names:
  brev create my-gpu | brev open cursor | brev exec 'pip install torch'

Only outputs when stdout is piped, keeping interactive use clean.

Author: theFong

docs/PRD-composable-cli.md

@@ -49,7 +49,7 @@ Make the Brev CLI idiomatic, programmable, and agent-friendly. Users and AI agen
 | `brev create` | Instance types (table or JSON) | Instance names | ✅ |
 | `brev shell` | - | - (interactive) | ✅ |
 | `brev exec` | Instance names | Command stdout/stderr | ✅ |
-| `brev open` | Instance names | - | ✅ |
+| `brev open` | Instance names | Instance names | ✅ |
 
 ### Exec Command (`brev exec`)
 
@@ -117,6 +117,13 @@ brev open gpu-1 gpu-2 gpu-3 cursor
 brev create my-cluster --count 3 | brev open cursor
 ```
 
+**Output for chaining**:
+Outputs instance names when piped, enabling pipelines:
+```bash
+# Create, open in editor, then run setup
+brev create my-gpu | brev open cursor | brev exec "pip install -r requirements.txt"
+```
+
 **Cross-platform support**:
 - macOS: Terminal.app, iTerm2
 - Linux: Default terminal emulator

pkg/cmd/open/open.go

@@ -169,6 +169,10 @@ func NewCmdOpen(t *terminal.Terminal, store OpenStore, noLoginStartStore OpenSto
 					}
 					return breverrors.WrapAndTrace(err)
 				}
+				// Output instance name for chaining (only if stdout is piped)
+				if isPiped() {
+					fmt.Println(instanceName)
+				}
 			}
 			if errors != nil {
 				return breverrors.WrapAndTrace(errors)
@@ -190,6 +194,12 @@ func isEditorType(s string) bool {
 	return s == EditorVSCode || s == EditorCursor || s == EditorWindsurf || s == EditorTerminal || s == EditorTmux
 }
 
+// isPiped returns true if stdout is piped to another command
+func isPiped() bool {
+	stat, _ := os.Stdout.Stat()
+	return (stat.Mode() & os.ModeCharDevice) == 0
+}
+
 // getInstanceNamesAndEditor gets instance names from args/stdin and determines editor type
 // editorFlag takes precedence, otherwise last arg may be an editor type (code, cursor, windsurf, tmux)
 func getInstanceNamesAndEditor(args []string, editorFlag string) ([]string, string, error) {