feat(ngrok): support fixed endpoint URL

[ridvanaltun]

Summary

Expose ngrok’s --url=… flag so users with a reserved endpoint or custom domain can pin the public URL instead of getting a random one each run.

Changes

• config.yaml: optional ngrok_url; passed to ngrok as --url=<value> when set.
• CLI: --ngrok-url <URL> (empty string clears / overrides to unset).
• NgrokTunnel: builds ngrok http <local-target> --url=… when configured.
• README: usage for config + CLI.
• Tests: config parsing, CLI flag, tunnel argv.

Usage

config.yaml:

ngrok: true
ngrok_url: https://your-subdomain.ngrok.dev

CLI:

deepseek-cursor-proxy --ngrok-url https://your-subdomain.ngrok.dev

[yxlao]

LGTM, thanks for the contribution!

Small nit: I changed “random URL each time” to “URL assigned by ngrok.” In my experience, even free-plan ngrok URLs/dev domains are usually stable across runs, so this wording is a bit more accurate.