#801 Stage 3 epic: agent specs + call_agent handoffs (#agentic)

ALTER highway.agents to carry a full run_goal spec (system_prompt, route, tool_catalog, tool_config, approval_policy, memory_namespace, max_turns, token_budget, output_schema, default_session_prefix); management.py CRUD additive (trigger-bundle intact); MCP run_goal(agent=NAME) resolves spec->DSL kwargs + create_agent/get_agent carry spec; call_agent catalog tool = named agent as child workflow run + durable join (reuse spawn_subgoals, expected=1), HITL-gated, depth<=1; reaction/scheduled agents read spec. Gates: harness + agent_spec_applied (narrow catalog+route in llm_call spans) + handoff (parent_run_id linkage + join).