Execute unified AgentStack batch (agentstack.execute)

Unified MCP entrypoint. Handles both standard MCP JSON-RPC 2.0 (Cursor streamable-http) and batch format with steps.

Batch format (steps):

• steps: array of { id, action, params?, if? }. Params may use refs: {"from": "stepId.result.field"} or {"from": "context.field"}.
• context (optional): object merged into request context for all steps. project_id and user_id override the session. locale and client_locale select this request's language and are not saved. Use this to override project/user without changing API key. Example: {"context": {"project_id": 123, "user_id": 456}}.
• If context.project_id or context.user_id is sent, they override session context; otherwise session (API key/JWT) is used.

Report (who got what, until when): If any step is buffs.apply_buff or buffs.apply_temporary_effect and succeeds, the response includes applied_buffs_summary: [{ buff_id, state, applied_at, expires_at, ... }] for buffs.apply_*, buffs.grant_subscription, and buffs.grant_tenant_subscription. Otherwise collect the same from step results manually (each step.result has applied_at, expires_at).