feat(cmd): add 'gt cat' command to display bead content

Implements the desire-path from bd-dcahx: agents naturally try
'gt cat <bead-id>' to view bead content, following Unix conventions.

The command validates bead ID prefixes (bd-*, hq-*, mol-*) and
delegates to 'bd show' for the actual display.

Supports --json flag for programmatic use.

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

internal/cmd/cat.go

@@ -0,0 +1,66 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+var catJSON bool
+
+var catCmd = &cobra.Command{
+	Use:     "cat <bead-id>",
+	GroupID: GroupWork,
+	Short:   "Display bead content",
+	Long: `Display the content of a bead (issue, task, molecule, etc.).
+
+This is a convenience wrapper around 'bd show' that integrates with gt.
+Accepts any bead ID (bd-*, hq-*, mol-*).
+
+Examples:
+  gt cat bd-abc123       # Show a bead
+  gt cat hq-xyz789       # Show a town-level bead
+  gt cat bd-abc --json   # Output as JSON`,
+	Args: cobra.ExactArgs(1),
+	RunE: runCat,
+}
+
+func init() {
+	rootCmd.AddCommand(catCmd)
+	catCmd.Flags().BoolVar(&catJSON, "json", false, "Output as JSON")
+}
+
+func runCat(cmd *cobra.Command, args []string) error {
+	beadID := args[0]
+
+	// Validate it looks like a bead ID
+	if !isBeadID(beadID) {
+		return fmt.Errorf("invalid bead ID %q (expected bd-*, hq-*, or mol-* prefix)", beadID)
+	}
+
+	// Build bd show command
+	bdArgs := []string{"show", beadID}
+	if catJSON {
+		bdArgs = append(bdArgs, "--json")
+	}
+
+	bdCmd := exec.Command("bd", bdArgs...)
+	bdCmd.Stdout = os.Stdout
+	bdCmd.Stderr = os.Stderr
+
+	return bdCmd.Run()
+}
+
+// isBeadID checks if a string looks like a bead ID.
+func isBeadID(s string) bool {
+	prefixes := []string{"bd-", "hq-", "mol-"}
+	for _, prefix := range prefixes {
+		if strings.HasPrefix(s, prefix) {
+			return true
+		}
+	}
+	return false
+}