Remove internal HMAC key auth from Copilot SDK docs (#61009)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sunbrye

content/copilot/how-tos/copilot-sdk/authenticate-copilot-sdk/authenticate-copilot-sdk.md

@@ -131,7 +131,6 @@ For complete setup instructions, including provider configuration options, limit
 When multiple authentication methods are available, the SDK uses them in this priority order:
 
 1. **Explicit `githubToken`** — Token passed directly to the SDK constructor
-1. **HMAC key** — `CAPI_HMAC_KEY` or `COPILOT_HMAC_KEY` environment variables
 1. **Direct API token** — `GITHUB_COPILOT_API_TOKEN` with `COPILOT_API_URL`
 1. **Environment variable tokens** — `COPILOT_GITHUB_TOKEN` → `GH_TOKEN` → `GITHUB_TOKEN`
 1. **Stored OAuth credentials** — From previous `copilot` CLI sign-in