Fix attachment path flexibility and file-link syncing in push

[rgonek]

Summary

• Allow push/validate to resolve attachment references for local files anywhere inside the space directory, with actionable diagnostics when a path escapes the space root.
• Support syncing non-image local file links ([]()) as attachments by including them in strict attachment indexing and reverse conversion.
• Normalize pushed attachment files into assets/<page-id>/..., update markdown links automatically, and emit ATTACHMENT_PATH_NORMALIZED diagnostics for moved paths.

Testing

• make lint
• make test