fix(copilot): switch to VS Code client ID and derive enterprise base URL

Two changes that complete the Copilot auth story (#7731 parts 3 and 4):

1. Switch OAuth client ID from opencode (Ov23li8tweQw6odWQebz) to VS Code
   (Iv1.b507a08c87ecfe98). The old ID produces gho_* tokens that return
   404 on /copilot_internal/v2/token, making token exchange non-functional.
   The new ID produces ghu_* tokens that support exchange.

2. Derive enterprise API base URL from the proxy-ep field in the exchanged
   token. Enterprise accounts get tokens containing e.g.
   "proxy-ep=proxy.enterprise.githubcopilot.com" which is converted to
   "https://api.enterprise.githubcopilot.com" and stored in the credential
   pool. Individual accounts (no proxy-ep) continue using the default URL.
   The COPILOT_API_BASE_URL env var remains as a user escape hatch.

Tested on both Individual and Enterprise Copilot accounts:
- Individual: device flow works, exchange succeeds, base_url=None (default)
- Enterprise: device flow works, exchange succeeds, 39 models returned
  including claude-opus-4.6-1m (936K), enterprise base URL derived

Parts 3 and 4 of #7731.

hermes_cli/auth.py

@@ -568,7 +568,8 @@ def _resolve_api_key_provider_secret(
             from hermes_cli.copilot_auth import resolve_copilot_token, get_copilot_api_token
             token, source = resolve_copilot_token()
             if token:
-                return get_copilot_api_token(token), source
+                api_token, _base_url = get_copilot_api_token(token)
+                return api_token, source
         except ValueError as exc:
             logger.warning("Copilot token validation failed: %s", exc)
         except Exception: