v0.7.0: Provision the agent lazily before a new session

AgentProvisioner::resolver() creates the agent when the schema has none and updates it when the system prompt or the tools changed, detected by a fingerprint stored on the mapping (AgentTrait::$fingerprint, new nullable column). An unchanged agent costs no API call, so a deploy needs neither an API key nor a call to Anthropic, and a broken key of one tenant fails only that tenant's chat. ensureProvisioned() keeps updating unconditionally unless force: false.