Add multi-repo config schema and source_repo metadata

For bd-307: Multi-repo hydration layer

Changes:
- Add MultiRepoConfig to internal/config
- Add GetMultiRepoConfig() to retrieve repos.primary and repos.additional
- Add source_repo field to Issue type to track ownership
- Prepare for hydration logic that reads from N repos

internal/config/config.go

@@ -167,3 +167,36 @@ func AllSettings() map[string]interface{} {
 	}
 	return v.AllSettings()
 }
+
+// GetStringSlice retrieves a string slice configuration value
+func GetStringSlice(key string) []string {
+	if v == nil {
+		return []string{}
+	}
+	return v.GetStringSlice(key)
+}
+
+// MultiRepoConfig contains configuration for multi-repo support
+type MultiRepoConfig struct {
+	Primary    string   // Primary repo path (where canonical issues live)
+	Additional []string // Additional repos to hydrate from
+}
+
+// GetMultiRepoConfig retrieves multi-repo configuration
+// Returns nil if multi-repo is not configured (single-repo mode)
+func GetMultiRepoConfig() *MultiRepoConfig {
+	if v == nil {
+		return nil
+	}
+	
+	// Check if repos.primary is set (indicates multi-repo mode)
+	primary := v.GetString("repos.primary")
+	if primary == "" {
+		return nil // Single-repo mode
+	}
+	
+	return &MultiRepoConfig{
+		Primary:    primary,
+		Additional: v.GetStringSlice("repos.additional"),
+	}
+}