h3 splitting in markdown import

[TomMaSS]

Summary

Support h3 (###) splitting in markdown import for nested section hierarchies, in addition to current h2 splitting.

Technical Approach

• Add heading_level parameter to prd_import_markdown (default: 2)
• When set to 3, split at h3 boundaries and nest under parent h2 sections
• Create parent-child section relationships via sort_order or a parent_id

Key Files

• mcp_server/tools/import_tools.py — prd_import_markdown

Acceptance Criteria

• heading_level parameter added to import tool
• h3 sections created as children of h2 sections
• Default behavior unchanged (h2 splitting)
• Unit test for nested import

[TomMaSS]

Implemented h3 splitting in markdown import. When heading_level >= 3, parent headings (one level above) create parent sections with parent_section_id linkage. Slug deduplication prevents collisions across repeated heading names. heading_level=1 and 2 behavior unchanged. Import wrapped in single transaction. Tests: parent+children creation, parent content, h2 unchanged, replace_existing updates parent linkage, slug dedupe.