Revises the Design section to describe the implemented solution:
provisioned_env on AgentProvisionPlan rather than an intermediate
egress_resolve_token_values_with_provider function. Drops the old
sentinel/lazy-import design narrative.
Extracts the forward_host_credentials / CODEX_HOST_CREDENTIAL_TOKEN_REF
resolution block, currently copy-pasted in both docker and smolmachines
launch files, into a single shared function in bot_bottle/egress.py.
Closes#118. Found via #117 hotspot review.