Add markdown support

[codegirl-007]

This adds markdown support for 99, enabling AI-assisted editing for documentation, technical writing, and mixed content workflows.

Changes

1. Language registration (state.lua) - Added "markdown" to supported languages
2. Language module (language/markdown.lua) - Basic language interface
3. Tree-sitter queries (queries/markdown/):
   • 99-function.scm - Treats headings and code blocks as editable units
   • 99-imports.scm - Extracts links as references
   • 99-scope.scm - Defines content boundaries

What It Enables

• 9v on a heading -> AI edits the entire section
• 9v in code block -> AI edits just the code
• 9s -> AI sees current section + all links

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[codegirl-007]

I don't know if I'm going to be the only one that finds this useful. Can take it or leave it. :)

[XandeCommits]

This would be very useful!