Add an offline `check` command (convert + validate a file, no network)

[willkg]

Add a check (a.k.a. lint/validate) subcommand that runs the converter on one or more markdown FILEs and reports problems without any network access or credentials — fast, safe, and CI/agent-friendly.

Why

This is the primitive an agent (Claude Code) or a pre-commit/CI loop wants: validate every doc edit instantly, offline, with no risk of mutating Confluence. Distinct from --dry-run (#11), which still contacts the server — create's validation calls ResolveSpaceID/PageExists/SearchPagesByTitle, so dry-run needs creds and a live space. check should need neither.

What it does

Run convert.MdToConfluence on each file and report:

• conversion warnings and broken image/link notices (already surfaced by the converter),
• frontmatter sanity (parseable; page_width ∈ narrow/wide/max; required fields present for the intended operation),
• local image/attachment paths that don't resolve on disk.

No client.Resolve, no HTTP. Composes with the existing --json envelope (a per-file result carrying warnings/broken), and exits non-zero if any file has errors.

Notes

• Base URL / space are needed by the converter for link rewriting; accept them as flags with harmless defaults (like the regression suite's https://wiki.example.net / ENG) so no live lookup is required.
• Reuses the internal/convert + warning/broken plumbing that already exists; mostly a thin command wrapper.
• Improves markfluence's use inside Claude Code: an agent can validate before it ever tries to publish.