LLM-based three-way merge of a user edit with the current LLM output

Semantic auto-merge for a single file.

Uses Claude to combine the user's saved content with the fresh LLM baseline: preserve user framing, absorb new facts. Saves the result as an updated content edit and returns the refreshed tree.

Only works on DRAFT trees that have an active content edit and a captured LLM baseline for the path.