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`.

feat: preview and confirm before push

`mdcx push` now prints the exact PUT request (URL, masked auth header,
title, version bump, body size delta, and a 20-line body excerpt) and
prompts for confirmation before sending. Add `--yes`/`-y` to skip the
prompt and `--dry-run` to preview only. Non-interactive stdin without
`--yes` errors out instead of hanging.

Splits `Client.UpdateContent` into `BuildUpdate` + send so the CLI can
render the same payload that will be transmitted.

Initial commit: mdcx — Markdown to Confluence XML converter

Bidirectional converter between Markdown and Confluence storage format XML
with self-hosted Confluence Server/Data Center sync support.

Features:
- convert: Markdown → Confluence XML
- extract: Confluence XML → Markdown
- embed: embed Markdown into Confluence XML template (marker-based)
- pull/push: sync pages with Confluence via REST API + PAT auth
- fmt: pretty-print Confluence XML with sensible indentation
- Round-trip preservation of inline comments, user refs, attachment images

false