feat: add self-update command

`felt update` checks GitHub releases for the latest version, downloads
the appropriate binary for the current OS/arch, and replaces the running
binary atomically.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cailmdaley

cmd/root.go

@@ -10,9 +10,13 @@ import (
 
 var jsonOutput bool
 
+// Version is the current version, set via ldflags.
+var Version = "dev"
+
 // SetVersionInfo sets version info from main (populated via ldflags)
-func SetVersionInfo(version, commit, date string) {
-	rootCmd.Version = version
+func SetVersionInfo(v, commit, date string) {
+	Version = v
+	rootCmd.Version = v
 }
 
 var rootCmd = &cobra.Command{

cmd/update.go

@@ -0,0 +1,161 @@
+package cmd
+
+import (
+	"archive/tar"
+	"compress/gzip"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"runtime"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+type ghRelease struct {
+	TagName string `json:"tag_name"`
+}
+
+func init() {
+	rootCmd.AddCommand(updateCmd)
+}
+
+var updateCmd = &cobra.Command{
+	Use:   "update",
+	Short: "Update felt to the latest version",
+	RunE: func(cmd *cobra.Command, args []string) error {
+		// Get latest release tag from GitHub
+		latest, err := latestVersion()
+		if err != nil {
+			return fmt.Errorf("checking latest version: %w", err)
+		}
+
+		current := Version
+		latestClean := strings.TrimPrefix(latest, "v")
+
+		if current == latestClean {
+			fmt.Printf("Already up to date (%s)\n", current)
+			return nil
+		}
+
+		if current == "dev" {
+			fmt.Println("Running a dev build — cannot determine current version.")
+			fmt.Printf("Latest release is %s. Continue? [y/N] ", latest)
+			var answer string
+			fmt.Scanln(&answer)
+			if answer != "y" && answer != "Y" {
+				return nil
+			}
+		} else {
+			fmt.Printf("Updating %s → %s\n", current, latestClean)
+		}
+
+		// Build asset name matching goreleaser template
+		assetName := fmt.Sprintf("felt_%s_%s.tar.gz", archiveOS(), archiveArch())
+		url := fmt.Sprintf("https://github.com/cailmdaley/felt/releases/download/%s/%s", latest, assetName)
+
+		// Download
+		resp, err := http.Get(url)
+		if err != nil {
+			return fmt.Errorf("downloading release: %w", err)
+		}
+		defer resp.Body.Close()
+		if resp.StatusCode != 200 {
+			return fmt.Errorf("download failed: %s (asset: %s)", resp.Status, assetName)
+		}
+
+		// Extract the "felt" binary from the tar.gz
+		binary, err := extractBinary(resp.Body)
+		if err != nil {
+			return fmt.Errorf("extracting binary: %w", err)
+		}
+
+		// Replace the running binary
+		exe, err := os.Executable()
+		if err != nil {
+			return fmt.Errorf("locating current binary: %w", err)
+		}
+
+		// Atomic-ish replace: rename old, write new, remove old
+		old := exe + ".old"
+		if err := os.Rename(exe, old); err != nil {
+			return fmt.Errorf("backing up current binary: %w (try running with sudo?)", err)
+		}
+
+		if err := os.WriteFile(exe, binary, 0755); err != nil {
+			// Try to restore
+			os.Rename(old, exe)
+			return fmt.Errorf("writing new binary: %w", err)
+		}
+
+		os.Remove(old)
+		fmt.Printf("Updated to %s\n", latestClean)
+		return nil
+	},
+}
+
+func latestVersion() (string, error) {
+	resp, err := http.Get("https://api.github.com/repos/cailmdaley/felt/releases/latest")
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return "", fmt.Errorf("GitHub API: %s", resp.Status)
+	}
+	var rel ghRelease
+	if err := json.NewDecoder(resp.Body).Decode(&rel); err != nil {
+		return "", err
+	}
+	return rel.TagName, nil
+}
+
+// archiveOS returns the OS name as goreleaser formats it (title case).
+func archiveOS() string {
+	switch runtime.GOOS {
+	case "darwin":
+		return "Darwin"
+	case "linux":
+		return "Linux"
+	default:
+		if len(runtime.GOOS) == 0 {
+			return ""
+		}
+		return strings.ToUpper(runtime.GOOS[:1]) + runtime.GOOS[1:]
+	}
+}
+
+// archiveArch returns the arch as goreleaser formats it.
+func archiveArch() string {
+	switch runtime.GOARCH {
+	case "amd64":
+		return "x86_64"
+	default:
+		return runtime.GOARCH
+	}
+}
+
+func extractBinary(r io.Reader) ([]byte, error) {
+	gz, err := gzip.NewReader(r)
+	if err != nil {
+		return nil, err
+	}
+	defer gz.Close()
+
+	tr := tar.NewReader(gz)
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+		if hdr.Name == "felt" || strings.HasSuffix(hdr.Name, "/felt") {
+			return io.ReadAll(tr)
+		}
+	}
+	return nil, fmt.Errorf("binary 'felt' not found in archive")
+}