Implement notio reindex — generate/update index

Implement notio reindex — generate/update index.md files for note type directories. Reference impl: pixecog:code/utils/cli/toc-index.py (parses YAML frontmatter, builds markdown tables with configurable columns and grouping). Should be either a notio_reindex(note_type?) MCP tool or auto-append on note_create. See issue-arash-20260326-185404 for full spec.

Related Notes

• [[issue-arash-20260326-185404-161144.md]] — Full spec for this task — directly referenced in the note as the source issue
• [[issue-arash-20260326-162053-576639.md]] — Related notio structural work — slim note types and add primitives, overlaps with reindex/index.md generation