feat: add amcs-cli — MCP tool CLI and stdio bridge

- cmd/amcs-cli: new CLI tool for human/AI MCP tool access
- amcs-cli tools: list all tools from remote MCP server
- amcs-cli call <tool> --arg k=v: invoke a tool, print JSON/YAML result
- amcs-cli stdio: stdio→HTTP MCP bridge for AI clients
- Config: ~/.config/amcs/config.yaml, AMCS_URL/AMCS_TOKEN env vars, --server/--token flags
- Token never logged in errors
- Makefile: add build-cli target

Closes #18

cmd/amcs-cli/cmd/config.go

@@ -0,0 +1,60 @@
+package cmd
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+)
+
+type Config struct {
+	Server string `yaml:"server"`
+	Token  string `yaml:"token"`
+}
+
+func defaultConfigPath() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("resolve home dir: %w", err)
+	}
+	return filepath.Join(home, ".config", "amcs", "config.yaml"), nil
+}
+
+func resolveConfigPath(path string) (string, error) {
+	if strings.TrimSpace(path) != "" {
+		return path, nil
+	}
+	return defaultConfigPath()
+}
+
+func loadConfigFile(path string) (Config, error) {
+	var cfg Config
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return cfg, nil
+		}
+		return cfg, fmt.Errorf("read config: %w", err)
+	}
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return cfg, fmt.Errorf("parse config: %w", err)
+	}
+	return cfg, nil
+}
+
+func saveConfigFile(path string, cfg Config) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return fmt.Errorf("create config dir: %w", err)
+	}
+	data, err := yaml.Marshal(cfg)
+	if err != nil {
+		return fmt.Errorf("marshal config: %w", err)
+	}
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		return fmt.Errorf("write config: %w", err)
+	}
+	return nil
+}