Filter out ghost cells during learning and when generating completions

[jlewi]

Filter out ghost cells

We don't want to include ghost cells as input when generating completions or as part of the doc that gets indexed when learning. This can lead to erroneous signal because the ghost cells could be wrong if they haven't been accepted (#174).
The bug is described in #174 (comment)

The fix is as follows

• Preserve metadata when converting from Runme proto to Foyle proto
• Metadata contains the _ghostCell key to indicate its a ghost cell
• Use a preprocessing function to filter out the ghost cells

We put the logic for filtering out ghostCells into a reusable preprocessing function so that we can use the logic consistently.

Fix #174

• We should preserve metadata when converting between blocks and cells
  • This will be useful for filtering out ghost cells
  • This will be useful for using AI to predict cell properties Set/Learn "interactive" mode and other cell configuration #157

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