feat: bake version into binary and display in UI sidebar

- Inject VERSION via ldflags (-X main=web.Version=) and git short SHA
- Add /api/version endpoint returning {version, commit}
- Display version in Layout sidebar footer (v0.1.6 · 8cfd405)
- Add AGENTS.md section documenting SemVer policy and bump-before-push rule
- Bump VERSION 0.1.5 -> 0.1.6 in Makefile

cmd/nasctl/main.go

@@ -17,6 +17,11 @@ import (
 	"github.com/darroyo/nasctl/internal/web"
 )
 
+var (
+	version = "dev"
+	commit  = ""
+)
+
 func main() {
 	addr := flag.String("addr", envOrDefault("NASCTL_ADDR", ":8080"), "HTTP listen address")
 	dbPath := flag.String("db", envOrDefault("NASCTL_DB", "/var/lib/nasctl/nasctl.db"), "SQLite database path")
@@ -79,6 +84,7 @@ func main() {
 	})
 
 	eng := engine.New(sambaModule, nfsModule, usersModule)
+	web.SetVersionInfo(version, commit)
 	srv := web.NewServer(database, eng, web.Options{
 		AllowedRoots: parseRoots(*allowedRoots),
 		Auth:         auth,
@@ -86,7 +92,7 @@ func main() {
 		ExportsPath:  *exportsPath,
 	})
 
-	log.Printf("nasctl listening on %s (db=%s exec-system=%v)", *addr, *dbPath, *execSystem)
+	log.Printf("nasctl %s (commit %s) listening on %s (db=%s exec-system=%v)", version, commit, *addr, *dbPath, *execSystem)
 	if err := http.ListenAndServe(*addr, srv.Handler()); err != nil {
 		log.Fatalf("server error: %v", err)
 	}