ACP provider credentials now route through cipher-protected agent_context.secrets, not acp_env

What this changes for operators

• Operators running ACP agents must understand provider API keys/base URLs now flow through the cipher-protected secrets channel; the deprecated acp_env channel no longer carries credentials.
• Changes the persistence and exposure surface for agent provider credentials, with SDK gap-fill logic specifically preventing re-folding into the insecure acp_env channel.
• Verification path: confirm ACP provider creds appear via agent_context.secrets and are absent from acp_env in agent context.