Implement multi-repo hydration layer with mtime caching (bd-307)

- Add repo_mtimes table to track JSONL file modification times
- Implement HydrateFromMultiRepo() with mtime-based skip optimization
- Support tilde expansion for repo paths in config
- Add source_repo column via migration (not in base schema)
- Fix schema to allow migration on existing databases
- Comprehensive test coverage for hydration logic
- Resurrect missing parent issues bd-cb64c226 and bd-cbed9619

Implementation:
- internal/storage/sqlite/multirepo.go - Core hydration logic
- internal/storage/sqlite/multirepo_test.go - Test coverage
- docs/MULTI_REPO_HYDRATION.md - Documentation

Schema changes:
- source_repo column added via migration only (not base schema)
- repo_mtimes table for mtime caching
- All SELECT queries updated to include source_repo

Database recovery:
- Restored from 17 to 285 issues
- Created placeholder parents for orphaned hierarchical children

Amp-Thread-ID: https://ampcode.com/threads/T-faa1339a-14b2-426c-8e18-aa8be6f5cde6
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/labels.go

@@ -98,7 +98,7 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT i.id, i.content_hash, i.title, i.description, i.design, i.acceptance_criteria, i.notes,
 		       i.status, i.priority, i.issue_type, i.assignee, i.estimated_minutes,
-		       i.created_at, i.updated_at, i.closed_at, i.external_ref
+		       i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo
 		FROM issues i
 		JOIN labels l ON i.id = l.issue_id
 		WHERE l.label = ?