Migrate from markdown executable to cmark Lua module

[leafo]

Summary

Replace the external markdown executable (discount 3.0.0d) with the cmark Lua module to reduce external dependencies.

Current Implementation

The codebase currently uses a build-time approach:

1. Build rule (static/md/Tupfile):

   : foreach *.md |> markdown < %f > %o |> %B.html
   

2. Markdown files in static/md/:

   • home.md
   • about.md
   • changes.md
   • intro.md
   • security_incident_march_2019.md

3. Views include pre-compiled HTML via raw_ssi helper in widgets/helpers.moon

Plan

Files to Modify

1. luarocks-site-dev-1.rockspec - Add cmark dependency
2. cmd/render_markdown.moon - New script to render markdown using cmark module
3. static/md/Tupfile - Update build rule to use new script

Benefits

• Removes dependency on external markdown executable
• Uses a Lua module that can be managed via LuaRocks
• cmark is the reference implementation for CommonMark, well-maintained by jgm

Tasks

• Add cmark to dependencies in rockspec
• Create cmd/render_markdown.moon script using cmark module
• Update static/md/Tupfile to use new script
• Test all pages that render markdown content

[leafo]

Consider using sitegen render command to process these files instead to have indexing and code highlighting support