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/types/types.go

@@ -29,6 +29,7 @@ type Issue struct {
 	CompactedAt        *time.Time     `json:"compacted_at,omitempty"`
 	CompactedAtCommit  *string        `json:"compacted_at_commit,omitempty"` // Git commit hash when compacted
 	OriginalSize       int            `json:"original_size,omitempty"`
+	SourceRepo         string         `json:"source_repo,omitempty"` // Which repo owns this issue (multi-repo support)
 	Labels             []string       `json:"labels,omitempty"` // Populated only for export/import
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 	Comments           []*Comment     `json:"comments,omitempty"`     // Populated only for export/import