Why: the backend prompt library ships new contract shapes and an
endpoint; the frontend mirror and api binding keep the TS side in sync
for upcoming management UI.
Evidence: tsc + vite build green as part of composite make test.
Impact: contracts.ts gains PromptModule/Trigger and
AgentProfile.promptModules; api gains listPromptModules. No UI change
yet.
Refs: feature/prompt-library task card.