Add install script, release Makefile and self-update

Releases are built locally (no runner) and uploaded to the Gitea generic
package registry. install.sh installs submarine plus a sub symlink;
`submarine update` replaces the binary after verifying its checksum.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

cmd/submarine/main.go

@@ -6,6 +6,8 @@ import (
 	"flag"
 	"fmt"
 	"os"
+	"path/filepath"
+	"time"
 
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/charmbracelet/lipgloss"
@@ -15,13 +17,27 @@ import (
 	"submarine/internal/submodule/git"
 	"submarine/internal/submodule/mock"
 	"submarine/internal/tui"
+	"submarine/internal/update"
 )
 
+// version is set at build time (-ldflags "-X main.version=v1.2.3").
+var version = "dev"
+
 func main() {
+	if len(os.Args) > 1 {
+		switch os.Args[1] {
+		case "update":
+			os.Exit(runUpdate(os.Args[2:]))
+		case "version", "--version", "-version":
+			fmt.Println("submarine", version)
+			return
+		}
+	}
+
 	useMock := flag.Bool("mock", false, "use built-in mock data instead of a real repository")
 	noFetch := flag.Bool("no-fetch", false, "don't fetch remotes in the background on startup")
 	flag.Usage = func() {
-		fmt.Fprintln(flag.CommandLine.Output(), "usage: submarine [--mock] [--no-fetch] [dir]")
+		fmt.Fprintln(flag.CommandLine.Output(), "usage: submarine [--mock] [--no-fetch] [dir]\n       submarine update [--check] [--force]\n       submarine version")
 		flag.PrintDefaults()
 	}
 	flag.Parse()
@@ -56,9 +72,75 @@ func main() {
 	zone.NewGlobal()
 	defer zone.Close()
 
+	notice := checkForUpdate(ctx, *noFetch || *useMock)
+
 	p := tea.NewProgram(tui.New(ctx, provider, tui.Options{Fetch: !*noFetch}), tea.WithAltScreen(), tea.WithMouseCellMotion())
 	if _, err := p.Run(); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
+	select {
+	case v := <-notice:
+		if v != "" {
+			fmt.Fprintf(os.Stderr, "submarine %s is available (you have %s), run: submarine update\n", v, version)
+		}
+	default:
+	}
+}
+
+// checkForUpdate looks for a newer release in the background (at most once a
+// day, cached) and delivers its version, or "" when there is none. Disabled
+// for dev builds, --no-fetch, --mock and SUBMARINE_NO_UPDATE_CHECK.
+func checkForUpdate(ctx context.Context, off bool) <-chan string {
+	out := make(chan string, 1)
+	cache, err := os.UserCacheDir()
+	if off || err != nil || !update.Release(version) || os.Getenv("SUBMARINE_NO_UPDATE_CHECK") != "" {
+		out <- ""
+		return out
+	}
+	go func() {
+		ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
+		defer cancel()
+		v, err := update.New().LatestCached(ctx, filepath.Join(cache, "submarine", "latest-version"))
+		if err != nil || !update.Newer(version, v) {
+			v = ""
+		}
+		out <- v
+	}()
+	return out
+}
+
+func runUpdate(args []string) int {
+	fs := flag.NewFlagSet("update", flag.ExitOnError)
+	check := fs.Bool("check", false, "only report whether a newer version exists")
+	force := fs.Bool("force", false, "install the latest version even if not newer (also for dev builds)")
+	fs.Parse(args)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Minute)
+	defer cancel()
+	c := update.New()
+	latest, err := c.Latest(ctx)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "submarine: update check failed:", err)
+		return 1
+	}
+	if !*force && !update.Newer(version, latest) {
+		fmt.Printf("submarine %s is up to date\n", version)
+		return 0
+	}
+	if *check {
+		fmt.Printf("update available: %s (you have %s)\n", latest, version)
+		return 0
+	}
+	if !update.Release(version) && !*force {
+		fmt.Fprintf(os.Stderr, "submarine: this is a %q build; use --force to replace it with %s\n", version, latest)
+		return 1
+	}
+	exe, err := c.Install(ctx, latest)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "submarine: update failed:", err)
+		return 1
+	}
+	fmt.Printf("updated %s: %s -> %s\n", exe, version, latest)
+	return 0
 }