Add a self-update command for the standalone PHAR

[WendellAdriel]

Overview

Standalone PHAR users have no way to update cpx short of manually downloading the latest release, while Composer-managed installs already have composer global update. This brings the standard self-update experience every PHAR-distributed CLI ships (Composer, Laravel Zero apps) to cpx.

Solution

cpx self-update fetches the latest GitHub release, verifies the downloaded PHAR against the sha256 digest the release API publishes, smoke-runs it with --version, and then swaps the running binary in place — an atomic rename on POSIX and a copy-over on Windows, with a backup restored if the swap fails. It follows the house conventions (--json envelope, Prompts output, streams-based HTTP with GITHUB_TOKEN support) and adds zero runtime dependencies. Composer-managed installs are updated too, with a note that composer global update cpx/cpx makes it stick.

Examples

$ cpx self-update

 ┌ cpx updated ─────────────────────────────────────┐
 │ Updated cpx from v1.9.9 to v2.0.0.               │
 └──────────────────────────────────────────────────┘

$ cpx self-update --json
{"success":true,"errors":[],"summary":{"updated":true,"from":"v1.9.9","to":"v2.0.0","path":"/usr/local/bin/cpx"}}