feat(cli): add multica update command (#218)

* feat(cli): add `multica update` command

Detects whether multica was installed via Homebrew (by resolving the
binary symlink and checking if it lives under a Homebrew prefix).

- Brew installs: runs `brew upgrade multica` automatically.
- Non-brew installs: prints instructions for installing via brew or
  downloading from GitHub releases.
- Checks latest version from the GitHub releases API and skips
  the update if already up to date.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(cli): use fully-qualified tap name in brew upgrade

Use `brew upgrade multica-ai/tap/multica` instead of `brew upgrade multica`
to avoid any potential name collision with core formulae.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/cmd/multica/cmd_update.go

@@ -0,0 +1,135 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/spf13/cobra"
+)
+
+var updateCmd = &cobra.Command{
+	Use:   "update",
+	Short: "Update multica to the latest version",
+	RunE:  runUpdate,
+}
+
+// githubRelease is the subset of the GitHub releases API response we need.
+type githubRelease struct {
+	TagName string `json:"tag_name"`
+	HTMLURL string `json:"html_url"`
+}
+
+func runUpdate(_ *cobra.Command, _ []string) error {
+	fmt.Fprintf(os.Stderr, "Current version: %s (commit: %s)\n", version, commit)
+
+	// Check latest version from GitHub.
+	latest, err := fetchLatestRelease()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Warning: could not check latest version: %v\n", err)
+	} else {
+		latestVer := strings.TrimPrefix(latest.TagName, "v")
+		currentVer := strings.TrimPrefix(version, "v")
+		if currentVer == latestVer {
+			fmt.Fprintln(os.Stderr, "Already up to date.")
+			return nil
+		}
+		fmt.Fprintf(os.Stderr, "Latest version:  %s\n\n", latest.TagName)
+	}
+
+	// Detect installation method and update accordingly.
+	if isBrewInstall() {
+		return updateViaBrew()
+	}
+
+	// Not installed via brew — show manual instructions.
+	fmt.Fprintln(os.Stderr, "multica was not installed via Homebrew.")
+	fmt.Fprintln(os.Stderr, "")
+	fmt.Fprintln(os.Stderr, "To install via Homebrew (recommended):")
+	fmt.Fprintln(os.Stderr, "  brew install multica-ai/tap/multica")
+	fmt.Fprintln(os.Stderr, "")
+	fmt.Fprintln(os.Stderr, "Or download the latest release from:")
+	fmt.Fprintln(os.Stderr, "  https://github.com/multica-ai/multica/releases/latest")
+	return nil
+}
+
+// isBrewInstall checks whether the running multica binary was installed via Homebrew.
+func isBrewInstall() bool {
+	exePath, err := os.Executable()
+	if err != nil {
+		return false
+	}
+	// Resolve symlinks (brew links binaries from Cellar into prefix/bin).
+	resolved, err := filepath.EvalSymlinks(exePath)
+	if err != nil {
+		resolved = exePath
+	}
+
+	// Check if the resolved path is inside a Homebrew prefix.
+	// Common prefixes: /opt/homebrew (Apple Silicon), /usr/local (Intel Mac), or custom.
+	brewPrefix := getBrewPrefix()
+	if brewPrefix != "" && strings.HasPrefix(resolved, brewPrefix) {
+		return true
+	}
+
+	// Fallback: check well-known Homebrew paths.
+	for _, prefix := range []string{"/opt/homebrew", "/usr/local", "/home/linuxbrew/.linuxbrew"} {
+		if strings.HasPrefix(resolved, prefix+"/Cellar/") {
+			return true
+		}
+	}
+	return false
+}
+
+// getBrewPrefix returns the Homebrew prefix by running `brew --prefix`, or empty string.
+func getBrewPrefix() string {
+	out, err := exec.Command("brew", "--prefix").Output()
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(out))
+}
+
+func updateViaBrew() error {
+	fmt.Fprintln(os.Stderr, "Updating via Homebrew...")
+
+	cmd := exec.Command("brew", "upgrade", "multica-ai/tap/multica")
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("brew upgrade failed: %w\nYou can try manually: brew upgrade multica-ai/tap/multica", err)
+	}
+
+	fmt.Fprintln(os.Stderr, "Update complete.")
+	return nil
+}
+
+func fetchLatestRelease() (*githubRelease, error) {
+	client := &http.Client{Timeout: 10 * time.Second}
+	req, err := http.NewRequest(http.MethodGet, "https://api.github.com/repos/multica-ai/multica/releases/latest", nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Accept", "application/vnd.github+json")
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("GitHub API returned %d", resp.StatusCode)
+	}
+
+	var release githubRelease
+	if err := json.NewDecoder(resp.Body).Decode(&release); err != nil {
+		return nil, err
+	}
+	return &release, nil
+}

server/cmd/multica/main.go

@@ -34,6 +34,7 @@ func init() {
 	rootCmd.AddCommand(issueCmd)
 	rootCmd.AddCommand(repoCmd)
 	rootCmd.AddCommand(versionCmd)
+	rootCmd.AddCommand(updateCmd)
 }
 
 func main() {