fix(copilot): exchange raw GitHub token for Copilot API JWT

Raw GitHub tokens (gho_/github_pat_/ghu_) are now exchanged for
short-lived Copilot API tokens via /copilot_internal/v2/token before
being used as Bearer credentials. This is required to access
internal-only models (e.g. claude-opus-4.6-1m with 1M context).

Implementation:
- exchange_copilot_token(): calls the token exchange endpoint with
  in-process caching (dict keyed by SHA-256 fingerprint), refreshed
  2 minutes before expiry. No disk persistence — gateway is long-running
  so in-memory cache is sufficient.
- get_copilot_api_token(): convenience wrapper with graceful fallback —
  returns exchanged token on success, raw token on failure.
- Both callers (hermes_cli/auth.py and agent/credential_pool.py) now
  pipe the raw token through get_copilot_api_token() before use.

12 new tests covering exchange, caching, expiry, error handling,
fingerprinting, and caller integration. All 185 existing copilot/auth
tests pass.

Part 2 of #7731.

agent/credential_pool.py

@@ -1078,9 +1078,10 @@ def _seed_from_singletons(provider: str, entries: List[PooledCredential]) -> Tup
         # env vars (COPILOT_GITHUB_TOKEN / GH_TOKEN).  They don't live in
         # the auth store or credential pool, so we resolve them here.
         try:
-            from hermes_cli.copilot_auth import resolve_copilot_token
+            from hermes_cli.copilot_auth import resolve_copilot_token, get_copilot_api_token
             token, source = resolve_copilot_token()
             if token:
+                api_token = get_copilot_api_token(token)
                 source_name = "gh_cli" if "gh" in source.lower() else f"env:{source}"
                 if not _is_suppressed(provider, source_name):
                     active_sources.add(source_name)
@@ -1092,7 +1093,7 @@ def _seed_from_singletons(provider: str, entries: List[PooledCredential]) -> Tup
                         {
                             "source": source_name,
                             "auth_type": AUTH_TYPE_API_KEY,
-                            "access_token": token,
+                            "access_token": api_token,
                             "base_url": pconfig.inference_base_url if pconfig else "",
                             "label": source,
                         },