Accept multiple markdown files as arguments

[masawada]

Why

`mdp` only took a single markdown file, so previewing related documents together meant running it once per file. Passing several files at once opens each in its own tab and lets `--watch` cover all of them.

What

• Accept every positional argument as a file path; check all files, dedupe by absolute path, and reject output path collisions before converting so a bad argument generates and opens nothing.
• Convert all files first, then open each output in the browser; `--watch` now watches every file with one fsnotify instance and regenerates only the changed one.
• Add unit tests for args, cli, and watcher, plus an e2e case that passes two files and checks both outputs and browser invocations.