Add `mex timeline --format md`

[theDakshJaitly]

Context

mex timeline currently renders to the terminal. Add a --format md option that emits Markdown, suitable for piping into reports / standup notes.

Scope

• Add a --format option to the timeline command (default: current terminal output; md: Markdown).
• Markdown output should be valid and readable (a table or a clean list of events).

Where

• timeline command in src/cli.ts and wherever timeline rendering lives (grep for timeline).

Acceptance criteria

• mex timeline --format md emits valid Markdown.
• Default output is unchanged.
• A test covers the md output shape.
• npm run typecheck && npm test && npm run build pass.