feat(migrate): add preview generation and markdown formatting

Implements suggest_path() for PARA-compliant destination paths with
VaultProfile support, generate_preview() that classifies all indexed
files and partitions into confident/uncertain buckets, and
format_preview_markdown() for human-readable review tables grouped by
category. Adds 16 new tests covering path suggestion, formatting output,
and edge cases.

Author: devwhodevs