feat: add comprehensive test suite and GoReleaser release automation

- Add GoReleaser configuration (`.goreleaser.yaml`)
  - Build for Linux, Windows, macOS (arm64, amd64, 386)
  - Enable UPX compression for Linux and Darwin binaries
  - Use version 2 syntax with custom archive naming
- Add Makefile with test targets
  - Run all tests via `make test-all` or just `make`
  - Individual test categories: `test-args`, `test-http`,
    `test-conversion`, `test-output`, `test-errors`, `test-build`
  - Also supports `build`, `clean`, and `help` targets
- Add integration test suite in `test/` directory
  - `test_args.go`: command-line flag parsing, help/version output
  - `test_http.go`: HTTP fetching, custom/default User-Agent, timeouts
  - `test_conversion.go`: HTML to Markdown conversion (headings, lists,
    links, code blocks, blockquotes, etc.)
  - `test_output.go`: file output and stdout behavior
  - `test_errors.go`: invalid URLs, timeouts, HTTP 4xx/5xx errors
  - `test_build.go`: end-to-end tests with compiled binary
  - `test_helper.go`: shared utilities (run commands, temp files,
    assertions)
- Add `test/README.md` documenting how to run the test suite
- Add `CHANGELOG.md` with [Unreleased] and [1.0.0] sections
- Update `.gitignore`
  - Remove platform‑specific binary patterns (`mdget*linux`,
    `mdget*x86_64`, etc.)
  - Add `dist/` (GoReleaser output directory)
- Update `README.md`

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