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.

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

feat: add verify command, improve round-trip fidelity

Add verify command for XML validation. Merge adjacent <code> elements
during XML→MD conversion. Preserve macro attribute order for round-trip
fidelity. Fix list item trailing whitespace and normalize <hr /> format.

false

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