Move CLI to cmd/mdcx/, add justfile and CLAUDE.md

Restructure to standard Go project layout: merge package cmd
into package main under cmd/mdcx/. Fix .gitignore to use /mdcx
so it only ignores the binary at repo root. Update install path
in README.md.

false

Rename module, add features, fix round-trip fidelity

- Rename module to sourcecraft.dev/bigbes/confluence-md-utilities
- Add fmt command with syntax highlighting (--color=auto/force/disabled)
- Add shell completions for all commands with file extension hints
- Preserve ac:layout/ac:layout-section/ac:layout-cell through round-trips
- Preserve ac:macro-id on code blocks and TOC macros
- Preserve table class/style attributes and colgroup column widths
- Preserve TOC macro as <!-- ac:toc --> comment
- Fix table cell rendering to preserve bold, italic, code, br, links
- Fix user references (ri:userkey) and attachment images (ri:attachment)
- Fix inline spacing between formatting elements in list items

false