fix(copilot): resolve GHE token poisoning when GITHUB_TOKEN is set

When GITHUB_TOKEN is present in the environment (e.g. for gh CLI or
GitHub Actions), two issues broke Copilot authentication against
GitHub Enterprise (GHE) instances:

1. The copilot provider had no base_url_env_var, so COPILOT_API_BASE_URL
   was silently ignored — requests always went to public GitHub.

2. `gh auth token` (the CLI fallback) treats GITHUB_TOKEN as an override
   and echoes it back instead of reading from its credential store
   (hosts.yml). This caused the same rejected token to be used even
   after env var priority correctly skipped it.

Fix:
- Add base_url_env_var="COPILOT_API_BASE_URL" to copilot ProviderConfig
- Strip GITHUB_TOKEN/GH_TOKEN from the subprocess env when calling
  `gh auth token` so it reads from hosts.yml
- Pass --hostname from COPILOT_GH_HOST when set so gh returns the
  GHE-specific OAuth token

hermes_cli/copilot_auth.py

@@ -117,14 +117,30 @@ def _gh_cli_candidates() -> list[str]:
 
 
 def _try_gh_cli_token() -> Optional[str]:
-    """Return a token from ``gh auth token`` when the GitHub CLI is available."""
+    """Return a token from ``gh auth token`` when the GitHub CLI is available.
+
+    When COPILOT_GH_HOST is set, passes ``--hostname`` so gh returns the
+    correct host's token.  Also strips GITHUB_TOKEN / GH_TOKEN from the
+    subprocess environment so ``gh`` reads from its own credential store
+    (hosts.yml) instead of just echoing the env var back.
+    """
+    hostname = os.getenv("COPILOT_GH_HOST", "").strip()
+
+    # Build a clean env so gh doesn't short-circuit on GITHUB_TOKEN / GH_TOKEN
+    clean_env = {k: v for k, v in os.environ.items()
+                 if k not in ("GITHUB_TOKEN", "GH_TOKEN")}
+
     for gh_path in _gh_cli_candidates():
+        cmd = [gh_path, "auth", "token"]
+        if hostname:
+            cmd += ["--hostname", hostname]
         try:
             result = subprocess.run(
-                [gh_path, "auth", "token"],
+                cmd,
                 capture_output=True,
                 text=True,
                 timeout=5,
+                env=clean_env,
             )
         except (FileNotFoundError, subprocess.TimeoutExpired) as exc:
             logger.debug("gh CLI token lookup failed (%s): %s", gh_path, exc)