Switch from Docker to composite action for cross-platform support

[SeanTAllen]

Docker container actions only run on Linux runners. Several ponylang repos have workflows with macOS and Windows jobs that need to send Zulip notifications on failure. The current Docker-based action can't be used in those jobs.

Switch from using: docker to using: composite in action.yml. The composite action would:

1. Detect the runner OS and architecture via $RUNNER_OS and $RUNNER_ARCH
2. Download the appropriate pre-built binary from the release assets
3. Run it

The release process would need to build and publish binaries for:

• linux-x86_64
• macos-x86_64
• macos-arm64
• windows-x86_64

Each tagged release's action.yml hardcodes the download URL pointing to its own release assets, so version resolution is automatic — ponylang/zulip-action@0.2.0 checks out the 0.2.0 tag, whose action.yml contains URLs like https://github.com/ponylang/zulip-action/releases/download/0.2.0/zulip-action-linux-x86_64.

This unblocks migrating all ponylang repos from zulip/github-actions-zulip to ponylang/zulip-action, including workflows that run on non-Linux runners.