Migrate from markdown executable to cmark Lua module

Nobody has claimed this yet.

## Summary

Replace the external `markdown` executable (discount 3.0.0d) with the [`cmark` Lua module](https://luarocks.org/modules/jgm/cmark) 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

Start with luarocks-site-dev-1.rockspec, cmd/render_markdown.moon, and static/md/Tupfile; compare the existing build rule with the planned cmark-based renderer. Rebuild the five files in static/md/ and check the generated pages, with completion meaning cmark is managed through LuaRocks and the pages render without the external markdown executable.

Written by the indexing model from the issue text.