Add Linux --setup wizard, service generation, and release workflow

[matveynator]

Motivation

• Provide a simple interactive installer for Linux hosts to configure the proxy (ask whether to use HTTPS, which domain, and where to forward requests).
• Make it easy to install and run the proxy as a service by auto-generating either systemd unit files or a SysV init.d script depending on the host.
• Improve operator discovery by writing runbook/service commands to /etc/info and provide a documented workflow.

Description

• Added a Linux-only pkg/setup package (build-tagged) exposing RegisterFlag() and RunInteractive() which implements an interactive setup that prompts for HTTPS usage, domain, and target URL and validates the target URL with url.Parse.
• The setup flow detects the init system with detectInitSystem() and writes either a systemd unit at /etc/systemd/system/chicha-http-proxy.service or an init.d script at /etc/init.d/chicha-http-proxy, and appends a timestamped runbook block to /etc/info with start/stop/status/log commands.
• Wired the CLI to register the Linux-only --setup flag and invoke setup.RunInteractive() early in main (in chicha-http-proxy.go) so invocation --setup runs the interactive installer and exits.
• Added a non-Linux stub pkg/setup/setup_stub.go so the --setup flag is not exposed on other platforms, and added docs/setup.md describing the flow.
• Added a GitHub Actions workflow at .github/workflows/release.yml to produce cross-platform artifacts when a commit message contains stable release.

Testing

• Ran go test ./... which completed successfully (packages report no test files and no failures).
• Ensured formatting with gofmt -w on modified files and ran go mod tidy to refresh module metadata (succeeded).

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Codex Task