Make gt witness attach infer rig from cwd when no arg provided

Align witness attach command with crew at and refinery attach by using
MaximumNArgs(1) instead of ExactArgs(1) and inferring the rig from the
current working directory when no argument is provided.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/witness.go

@@ -13,6 +13,7 @@ import (
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/tmux"
 	"github.com/steveyegge/gastown/internal/witness"
+	"github.com/steveyegge/gastown/internal/workspace"
 )
 
 // Witness command flags
@@ -67,7 +68,7 @@ Displays running state, monitored polecats, and statistics.`,
 }
 
 var witnessAttachCmd = &cobra.Command{
-	Use:     "attach <rig>",
+	Use:     "attach [rig]",
 	Aliases: []string{"at"},
 	Short:   "Attach to witness session",
 	Long: `Attach to the Witness tmux session for a rig.
@@ -75,8 +76,13 @@ var witnessAttachCmd = &cobra.Command{
 Attaches the current terminal to the witness's tmux session.
 Detach with Ctrl-B D.
 
-If the witness is not running, this will start it first.`,
-	Args: cobra.ExactArgs(1),
+If the witness is not running, this will start it first.
+If rig is not specified, infers it from the current directory.
+
+Examples:
+  gt witness attach gastown
+  gt witness attach          # infer rig from cwd`,
+	Args: cobra.MaximumNArgs(1),
 	RunE: runWitnessAttach,
 }
 
@@ -342,7 +348,22 @@ func ensureWitnessSession(rigName string, r *rig.Rig) (bool, error) {
 }
 
 func runWitnessAttach(cmd *cobra.Command, args []string) error {
-	rigName := args[0]
+	rigName := ""
+	if len(args) > 0 {
+		rigName = args[0]
+	}
+
+	// Infer rig from cwd if not provided
+	if rigName == "" {
+		townRoot, err := workspace.FindFromCwdOrError()
+		if err != nil {
+			return fmt.Errorf("not in a Gas Town workspace: %w", err)
+		}
+		rigName, err = inferRigFromCwd(townRoot)
+		if err != nil {
+			return fmt.Errorf("could not determine rig: %w\nUsage: gt witness attach <rig>", err)
+		}
+	}
 
 	// Verify rig exists
 	_, r, err := getWitnessManager(rigName)