feat(01-02): provider YAML schema structs with validation and reference YAML files

- Provider, Pattern, VerifySpec, RegistryStats structs in schema.go
- UnmarshalYAML validates format_version >= 1 and last_verified non-empty
- Three reference YAML files: openai, anthropic, huggingface

pkg/providers/schema.go

@@ -0,0 +1,66 @@
+package providers
+
+import (
+	"fmt"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Provider represents a single API key provider definition loaded from YAML.
+type Provider struct {
+	FormatVersion int        `yaml:"format_version"`
+	Name          string     `yaml:"name"`
+	DisplayName   string     `yaml:"display_name"`
+	Tier          int        `yaml:"tier"`
+	LastVerified  string     `yaml:"last_verified"`
+	Keywords      []string   `yaml:"keywords"`
+	Patterns      []Pattern  `yaml:"patterns"`
+	Verify        VerifySpec `yaml:"verify"`
+}
+
+// Pattern defines a single regex pattern for API key detection.
+type Pattern struct {
+	Regex      string  `yaml:"regex"`
+	EntropyMin float64 `yaml:"entropy_min"`
+	Confidence string  `yaml:"confidence"`
+}
+
+// VerifySpec defines how to verify a key is live (used by Phase 5 verification engine).
+type VerifySpec struct {
+	Method        string            `yaml:"method"`
+	URL           string            `yaml:"url"`
+	Headers       map[string]string `yaml:"headers"`
+	ValidStatus   []int             `yaml:"valid_status"`
+	InvalidStatus []int             `yaml:"invalid_status"`
+}
+
+// RegistryStats holds aggregate statistics about loaded providers.
+type RegistryStats struct {
+	Total        int
+	ByTier       map[int]int
+	ByConfidence map[string]int
+}
+
+// UnmarshalYAML implements yaml.Unmarshaler with schema validation (satisfies PROV-10).
+func (p *Provider) UnmarshalYAML(value *yaml.Node) error {
+	// Use a type alias to avoid infinite recursion
+	type ProviderAlias Provider
+	var alias ProviderAlias
+	if err := value.Decode(&alias); err != nil {
+		return err
+	}
+	if alias.FormatVersion < 1 {
+		return fmt.Errorf("provider %q: format_version must be >= 1 (got %d)", alias.Name, alias.FormatVersion)
+	}
+	if alias.LastVerified == "" {
+		return fmt.Errorf("provider %q: last_verified is required", alias.Name)
+	}
+	validConfidences := map[string]bool{"high": true, "medium": true, "low": true, "": true}
+	for _, pat := range alias.Patterns {
+		if !validConfidences[pat.Confidence] {
+			return fmt.Errorf("provider %q: pattern confidence %q must be high, medium, or low", alias.Name, pat.Confidence)
+		}
+	}
+	*p = Provider(alias)
+	return nil
+}