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.
