chore: rename module to go.bigb.es/confluence-md-utilities

feat: add ability for local install

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.

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