Add `mex export` (bundle scaffold to a single Markdown file)

[theDakshJaitly]

Context

Add a mex export command that bundles the whole scaffold into a single Markdown file — for pasting into tools that don't read files well.

Scope

• New mex export command that concatenates the scaffold files (ROUTER.md, context/, patterns/, etc.) into one Markdown document, with a clear section header per source file.
• Output to stdout by default; optionally to a path.

Where

• New command in src/cli.ts.
• Reuse scaffold-file discovery — see DEFAULT_SCAFFOLD_PATTERNS and findScaffoldFiles in src/drift/index.ts.

Acceptance criteria

• mex export produces a single Markdown file containing all scaffold files, each under a header.
• Works with stdout and (if implemented) a target path.
• A test covers the bundled output.
• npm run typecheck && npm test && npm run build pass.