Lifts bottle.tokens into a per-route CredProxyUpstream table, renders a
mode-600 routes.json that carries no token values or host env-var
names, and derives the {token_env: TokenRef} map the launch step will
use to forward host env values into the sidecar's environ.
Shape mirrors GitGate/PipelockProxy: abstract base does the host-side
prepare; start/stop is backend-specific. No backend wiring yet.
