feat: add pull --with-comments sidecar

`mdcx pull --with-comments` fetches inline + page comments via
`/rest/api/content/{id}/child/comment` (paginated, two-level reply
expansion) and writes them to `<output>.comments.json` alongside the
Markdown file. Inline comments correlate to the markdown's
`data-inline-comment="UUID"` spans through their `marker_ref` field.

Comment bodies are stored as raw Confluence storage XML; pushing
comments back is not implemented. Requires `--output`.

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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