feat: initial commit of mdget

- Add core implementation (`cmd/mdget.go`)
  - Fetch webpages with configurable timeout and User-Agent
  - Convert HTML to Markdown using `html-to-markdown/v2`
  - Support output to stdout or file via `-o/--output`
  - Automatically prepend `https://` to bare domains
  - Provide `--version` and `--help` via `go-arg`

- Add project metadata
  - `go.mod` / `go.sum` with dependencies: `req/v3`,
    `html-to-markdown/v2`, `go-arg`
  - `LICENCE.txt` (EUPL 1.2) with full legal text
  - `README.md` with usage, examples, build instructions

- Add assets
  - `assets/logo.png` and `assets/screenshot.png`
  - `assets/eupl-12-badge.svg` for licence badge

- Configure `.gitignore` for Go binaries, test artifacts, coverage, and
  environment files