feat(stale): add gt stale command for binary staleness check

Exposes CheckStaleBinary() via CLI for scripting. Supports --json for
machine-readable output and --quiet for exit-code-only mode (0=stale,
1=fresh, 2=error).

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

internal/cmd/stale.go

@@ -0,0 +1,122 @@
+package cmd
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/style"
+	"github.com/steveyegge/gastown/internal/version"
+)
+
+var staleJSON bool
+var staleQuiet bool
+
+var staleCmd = &cobra.Command{
+	Use:     "stale",
+	GroupID: GroupDiag,
+	Short:   "Check if the gt binary is stale",
+	Long: `Check if the gt binary was built from an older commit than the current repo HEAD.
+
+This command compares the commit hash embedded in the binary at build time
+with the current HEAD of the gastown repository.
+
+Examples:
+  gt stale              # Human-readable output
+  gt stale --json       # Machine-readable JSON output
+  gt stale --quiet      # Exit code only (0=stale, 1=fresh)
+
+Exit codes:
+  0 - Binary is stale (needs rebuild)
+  1 - Binary is fresh (up to date)
+  2 - Error (could not determine staleness)`,
+	RunE: runStale,
+}
+
+func init() {
+	staleCmd.Flags().BoolVar(&staleJSON, "json", false, "Output as JSON")
+	staleCmd.Flags().BoolVarP(&staleQuiet, "quiet", "q", false, "Exit code only (0=stale, 1=fresh)")
+	rootCmd.AddCommand(staleCmd)
+}
+
+// StaleOutput represents the JSON output structure.
+type StaleOutput struct {
+	Stale         bool   `json:"stale"`
+	BinaryCommit  string `json:"binary_commit"`
+	RepoCommit    string `json:"repo_commit"`
+	CommitsBehind int    `json:"commits_behind,omitempty"`
+	Error         string `json:"error,omitempty"`
+}
+
+func runStale(cmd *cobra.Command, args []string) error {
+	// Find the gastown repo
+	repoRoot, err := version.GetRepoRoot()
+	if err != nil {
+		if staleQuiet {
+			os.Exit(2)
+		}
+		if staleJSON {
+			return outputStaleJSON(StaleOutput{Error: err.Error()})
+		}
+		return fmt.Errorf("cannot find gastown repo: %w", err)
+	}
+
+	// Check staleness
+	info := version.CheckStaleBinary(repoRoot)
+
+	// Handle errors
+	if info.Error != nil {
+		if staleQuiet {
+			os.Exit(2)
+		}
+		if staleJSON {
+			return outputStaleJSON(StaleOutput{Error: info.Error.Error()})
+		}
+		return fmt.Errorf("staleness check failed: %w", info.Error)
+	}
+
+	// Quiet mode: just exit with appropriate code
+	if staleQuiet {
+		if info.IsStale {
+			os.Exit(0)
+		}
+		os.Exit(1)
+	}
+
+	// Build output
+	output := StaleOutput{
+		Stale:         info.IsStale,
+		BinaryCommit:  info.BinaryCommit,
+		RepoCommit:    info.RepoCommit,
+		CommitsBehind: info.CommitsBehind,
+	}
+
+	if staleJSON {
+		return outputStaleJSON(output)
+	}
+
+	return outputStaleText(output)
+}
+
+func outputStaleJSON(output StaleOutput) error {
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "  ")
+	return enc.Encode(output)
+}
+
+func outputStaleText(output StaleOutput) error {
+	if output.Stale {
+		fmt.Printf("%s Binary is stale\n", style.Warning.Render("⚠"))
+		fmt.Printf("  Binary: %s\n", version.ShortCommit(output.BinaryCommit))
+		fmt.Printf("  Repo:   %s\n", version.ShortCommit(output.RepoCommit))
+		if output.CommitsBehind > 0 {
+			fmt.Printf("  %s\n", style.Dim.Render(fmt.Sprintf("(%d commits behind)", output.CommitsBehind)))
+		}
+		fmt.Printf("\n  Run 'go install ./cmd/gt' to rebuild\n")
+	} else {
+		fmt.Printf("%s Binary is fresh\n", style.Success.Render("✓"))
+		fmt.Printf("  Commit: %s\n", version.ShortCommit(output.BinaryCommit))
+	}
+	return nil
+}