feat(cli): add mex export (bundle scaffold to a single Markdown file)

[abhinav-phi]

Resolves #56.

What

mex export concatenates the whole scaffold into one Markdown document, for pasting into tools that don't read files well:

• Same discovery as mex check — reuses findScaffoldFiles + DEFAULT_SCAFFOLD_PATTERNS, so the export is exactly what the drift scanner sees, never a divergent file list.
• Each file lands under a ## <scaffold-relative-path> header, with content trimmed of trailing whitespace so the document stays clean.
• Deterministic order (sorted by path), so repeated exports diff cleanly.
• mex export → stdout; mex export --out exports/scaffold.md → file (parent directories created) plus a one-line count report.
• An empty scaffold fails with No scaffold files found. Run: mex setup.

Tests

Three cases in test/export.test.ts: full bundle (headers, content, deterministic order), --out write + count report, and missing-scaffold guidance. npm run typecheck green.

[abhinav-phi]

CI caught this PR violating the wiki-architecture write pin: src/export.ts introduces a writeFileSync outside src/wiki/, so the pinned-writers test counted 15 writers against the allowlist's 14. Registered the writer with its exemption — the --out path is a brand-new file the user names, which is the class the allowlist's own comment describes ("write JSON, hooks, or brand-new files, so there are no bytes of anybody's to preserve"). No production change needed; the test now documents the export writer's scope like every other entry.