## Why
The existing loopback listener mode does not provide the stdin/stdout transport needed by OpenSSH's `ProxyCommand`.
## What changed
- Relay one SSH session over HTTP/3 CONNECT with `--environment-id ENV --port PORT`, defaulting to the public Portals gateway and port 22. Also accept an explicit `--proxy-url` with `--target ENV:PORT`.
- Require ChatGPT OAuth login, enforce cloud configuration and application network policy, and restrict OAuth credentials to the built-in gateway origin. Leave SSH authentication and host-key verification to OpenSSH.
- Preserve binary stdio, propagate input half-close, exit on gateway EOF, and report rejected CONNECT requests by HTTP status without exposing arbitrary peer error text.
- Require ERS-issued v2 tunnel JWTs for Portals listener mode unless OAuth mode is selected, and reject older ERS tunnel JWTs during credential updates.
- Disable UDP segmentation offload for SSH tunnels on Linux to avoid stalled bulk transfers on affected paths. Document an OpenSSH configuration example.
## Testing
Add coverage for argument validation, gateway routing, credential boundaries, JWT startup and renewal checks, binary relay and half-close behavior, gateway EOF and rejection, and cloud policy enforcement before gateway connection.
GitOrigin-RevId: 0d9afbddef01748efcc1c4b88f7e19923deff7e1
Anton Panasenko
c82708f858