Security: Prevent gateway credential exfiltration via URL override (#9179)

* Gateway: require explicit auth for url overrides

* Gateway: scope credential blocking to non-local URLs only

Address review feedback: the previous fix blocked credential fallback for
ALL URL overrides, which was overly strict and could break workflows that
use --url to switch between loopback/tailnet without passing credentials.

Now credential fallback is only blocked for non-local URLs (public IPs,
external hostnames). Local addresses (127.0.0.1, localhost, private IPs
like 192.168.x.x, 10.x.x.x, tailnet 100.x.x.x) still get credential
fallback as before.

This maintains the security fix (preventing credential exfiltration to
attacker-controlled URLs) while preserving backward compatibility for
legitimate local URL overrides.

* Security: require explicit credentials for gateway url overrides (#8113) (thanks @victormier)

* Gateway: reuse explicit auth helper for url overrides (#8113) (thanks @victormier)

* Tests: format gateway chat test (#8113) (thanks @victormier)

* Tests: require explicit auth for gateway url overrides (#8113) (thanks @victormier)

---------

Co-authored-by: Victor Mier <victormier@gmail.com>

docs/tui.md

@@ -142,6 +142,9 @@ Other Gateway slash commands (for example, `/context`) are forwarded to the Gate
 - `--thinking <level>`: Override thinking level for sends
 - `--timeout-ms <ms>`: Agent timeout in ms (defaults to `agents.defaults.timeoutSeconds`)
 
+Note: when you set `--url`, the TUI does not fall back to config or environment credentials.
+Pass `--token` or `--password` explicitly. Missing explicit credentials is an error.
+
 ## Troubleshooting
 
 No output after sending a message: