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