shark expose: put a local port on the internet through Hammerhead

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

internal/config/config.go

@@ -0,0 +1,66 @@
+// Package config keeps the Hammerhead address and token between runs.
+package config
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+type Config struct {
+	Hammerhead string `json:"hammerhead"`
+	Token      string `json:"token"`
+}
+
+// Path is where `shark login` saves the settings; SHARK_CONFIG overrides it.
+func Path() (string, error) {
+	if p := os.Getenv("SHARK_CONFIG"); p != "" {
+		return p, nil
+	}
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, "shark", "config.json"), nil
+}
+
+// Load reads the saved settings, then lets SHARK_HAMMERHEAD and SHARK_TOKEN override them.
+// A missing file is not an error.
+func Load() (Config, error) {
+	var c Config
+	p, err := Path()
+	if err == nil {
+		b, rerr := os.ReadFile(p)
+		switch {
+		case rerr == nil:
+			if err := json.Unmarshal(b, &c); err != nil {
+				return c, errors.New(p + " is not valid JSON: " + err.Error())
+			}
+		case !errors.Is(rerr, os.ErrNotExist):
+			return c, rerr
+		}
+	}
+	if v := os.Getenv("SHARK_HAMMERHEAD"); v != "" {
+		c.Hammerhead = v
+	}
+	if v := os.Getenv("SHARK_TOKEN"); v != "" {
+		c.Token = v
+	}
+	c.Hammerhead = strings.TrimRight(c.Hammerhead, "/")
+	return c, nil
+}
+
+// Save writes the settings so that only the user can read them (the token is a credential).
+func Save(c Config) (string, error) {
+	p, err := Path()
+	if err != nil {
+		return "", err
+	}
+	if err := os.MkdirAll(filepath.Dir(p), 0o700); err != nil {
+		return "", err
+	}
+	b, _ := json.MarshalIndent(c, "", "  ")
+	return p, os.WriteFile(p, append(b, '\n'), 0o600)
+}