feat: add Djot and Org-mode format handlers

Add two new format handlers using established Rust crates:
- djot.rs: Djot format using jotdown crate (handles sections, headings,
  paragraphs, code blocks, lists, tables, admonitions, footnotes)
- orgmode.rs: Org-mode format using orgize crate (handles titles,
  source blocks, quote blocks, lists, tables, keywords)

Both handlers implement Parser, Renderer, and FormatHandler traits
for full round-trip conversion support. All 12 tests pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell