refactor: switch config format from YAML to TOML

- Replace gopkg.in/yaml.v3 with github.com/BurntSushi/toml
- Update struct tags from yaml to toml
- Update config tests to use TOML syntax
- Add testdata/config.toml, update Helm configmap template

internal/config/config.go

@@ -6,39 +6,39 @@ import (
 	"regexp"
 	"strings"
 
-	"gopkg.in/yaml.v3"
+	"github.com/BurntSushi/toml"
 )
 
 type Config struct {
-	Server ServerConfig `yaml:"server"`
-	Git    GitConfig    `yaml:"git"`
-	Cluster ClusterConfig `yaml:"cluster"`
-	ToolchainRegistry string `yaml:"toolchain_registry"`
-	TargetRepoTemplate string `yaml:"target_repo_template"`
-	Plugins []PluginConfig `yaml:"plugins"`
+	Server             ServerConfig   `toml:"server"`
+	Git                GitConfig      `toml:"git"`
+	Cluster            ClusterConfig  `toml:"cluster"`
+	ToolchainRegistry  string         `toml:"toolchain_registry"`
+	TargetRepoTemplate string         `toml:"target_repo_template"`
+	Plugins            []PluginConfig `toml:"plugins"`
 }
 
 type ServerConfig struct {
-	Addr      string `yaml:"addr"`
-	PublicURL string `yaml:"public_url"`
+	Addr      string `toml:"addr"`
+	PublicURL string `toml:"public_url"`
 }
 
 type GitConfig struct {
-	Type    string `yaml:"type"`
-	BaseURL string `yaml:"base_url"`
-	Token   string `yaml:"token"`
+	Type    string `toml:"type"`
+	BaseURL string `toml:"base_url"`
+	Token   string `toml:"token"`
 }
 
 type ClusterConfig struct {
-	Kubeconfig     string `yaml:"kubeconfig"`
-	Namespace      string `yaml:"namespace"`
-	JobTimeout     int    `yaml:"job_timeout"`
-	MaxConcurrent  int    `yaml:"max_concurrent"`
+	Kubeconfig    string `toml:"kubeconfig"`
+	Namespace     string `toml:"namespace"`
+	JobTimeout    int    `toml:"job_timeout"`
+	MaxConcurrent int    `toml:"max_concurrent"`
 }
 
 type PluginConfig struct {
-	Language string `yaml:"language"`
-	Image    string `yaml:"image"`
+	Language string `toml:"language"`
+	Image    string `toml:"image"`
 }
 
 var envVarRegex = regexp.MustCompile(`\$\{(\w+)\}`)
@@ -52,7 +52,7 @@ func Load(path string) (*Config, error) {
 	expanded := expandEnvVars(string(data))
 
 	var cfg Config
-	if err := yaml.Unmarshal([]byte(expanded), &cfg); err != nil {
+	if _, err := toml.Decode(expanded, &cfg); err != nil {
 		return nil, fmt.Errorf("parsing config: %w", err)
 	}