ci: add release binary automation and installer

[paoloanzn]

Summary

• add a GitHub Actions release workflow that cross-compiles flare-edge-cli for Linux, macOS, and Windows on amd64 and arm64
• package each build as a release archive, attach the archives to the published GitHub release, and generate a checksums.txt file
• add a portable install.sh script that detects the current platform, downloads the latest matching release asset, and installs the binary into a user-local bin directory
• update the README with a one-line install command, supported release targets, and release download guidance

Details

The new workflow runs on GitHub release.published events and produces release artifacts for:

• linux/amd64
• linux/arm64
• darwin/amd64
• darwin/arm64
• windows/amd64
• windows/arm64

Unix targets are published as .tar.gz archives and Windows targets as .zip archives. Each archive contains the compiled binary plus README.md and LICENSE, which makes the release assets self-contained.

The installer script is intentionally simple and shell-only. It:

• resolves a sensible install directory using INSTALL_DIR, XDG_BIN_HOME, ~/.local/bin, or ~/bin
• detects linux vs darwin and amd64 vs arm64
• fetches the latest GitHub release tag from the GitHub API
• downloads the matching archive for the current platform
• installs flare-edge-cli with executable permissions
• prints a PATH hint when the chosen install directory is not already on PATH

Testing

• sh -n install.sh

Notes

I did not run the release workflow end to end locally because asset publication depends on a real GitHub release event. The workflow is wired so that the next published release will exercise the full archive-and-attach path.