Function apply_workspace_rewrite 

pub fn apply_workspace_rewrite(
    conv: &mut NormalizedConversation,
    root: &ScanRoot,
)

Apply workspace path rewriting to a conversation.

This rewrites workspace paths from remote formats to local equivalents at ingest time, ensuring that workspace filters work consistently across local and remote sources.

The original workspace path is preserved in metadata.cass.workspace_original for display/audit purposes.

Part of P6.2 - Apply path mappings at ingest time.