Complete bd-ar2 P2 tasks: metadata, resurrection, and testing improvements

This commit addresses the remaining P2 tasks from bd-ar2 code review follow-up:

## Completed Tasks

### bd-ar2.4: Improve parent chain resurrection
- Modified `tryResurrectParentWithConn()` to recursively resurrect ancestor chain
- When resurrecting bd-root.1.2, now also resurrects bd-root.1 if missing
- Handles deeply nested hierarchies where intermediate parents are deleted
- All resurrection tests pass including new edge cases

### bd-ar2.5: Add error handling guidance
- Documented metadata update failure strategy in `updateExportMetadata()`
- Explained trade-off: warnings vs errors (safe, prevents data loss)
- Added user-facing message: "Next export may require running 'bd import' first"
- Clarifies that worst case is requiring import before next export

### bd-ar2.6: Document transaction boundaries
- Added comprehensive documentation for atomicity trade-offs
- Explained crash scenarios and recovery (bd import)
- Documented decision to defer defensive checks (Option 4) until needed
- No code changes - current approach is acceptable for now

### bd-ar2.12: Add metadata key validation
- Added keySuffix validation in `updateExportMetadata()` and `hasJSONLChanged()`
- Prevents ':' separator in keySuffix to avoid malformed metadata keys
- Documented metadata key format in function comments
- Single-repo: "last_import_hash", multi-repo: "last_import_hash:<repo_key>"

### bd-ar2.7: Add edge case tests for GetNextChildID resurrection
- TestGetNextChildID_ResurrectParent_NotInJSONL: parent not in history
- TestGetNextChildID_ResurrectParent_NoJSONL: missing JSONL file
- TestGetNextChildID_ResurrectParent_MalformedJSONL: invalid JSON lines
- TestGetNextChildID_ResurrectParentChain: deeply nested missing parents
- All tests pass, resurrection is robust against edge cases

## Files Changed
- cmd/bd/daemon_sync.go: Metadata validation, error handling docs
- cmd/bd/integrity.go: Added strings import, keySuffix validation
- internal/storage/sqlite/hash_ids.go: Improved resurrection comments
- internal/storage/sqlite/resurrection.go: Recursive ancestor resurrection
- internal/storage/sqlite/child_id_test.go: Added 4 new edge case tests

## Testing
All export, sync, metadata, and resurrection tests pass.
Edge cases properly handled: missing JSONL, malformed JSON, deep nesting.

## Remaining Tasks
- bd-ar2.8 (P3): Additional export metadata edge case tests (deferred)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/hash_ids.go

@@ -34,7 +34,11 @@ func (s *SQLiteStorage) GetNextChildID(ctx context.Context, parentID string) (st
 		return "", fmt.Errorf("failed to check parent existence: %w", err)
 	}
 	if count == 0 {
-		// Try to resurrect parent from JSONL history before failing (bd-dvd fix)
+		// Try to resurrect parent from JSONL history before failing (bd-dvd fix, bd-ar2.4)
+		// Note: Using TryResurrectParent instead of TryResurrectParentChain because we're
+		// already given the direct parent ID. TryResurrectParent will handle the direct parent,
+		// and if the parent itself has missing ancestors, those should have been resurrected
+		// when the parent was originally created.
 		resurrected, resurrectErr := s.TryResurrectParent(ctx, parentID)
 		if resurrectErr != nil {
 			return "", fmt.Errorf("failed to resurrect parent %s: %w", parentID, resurrectErr)