#805 Stage 6 epic: agent progress streaming (SSE) (#agentic)

SQL trigger NOTIFY on highway.agent_event insert (migration; engine untouched -> harness-gated). New SSE endpoint /workflows/<id>/agent-stream in api/blueprints/v1/stream.py (LISTEN + incremental replay of agent_event rows: turn, llm_call cost, tool_call, approval pending, done); existing workflow-status SSE is the template. MCP optional wait mode on run_goal via FastMCP Context.report_progress (fire-and-forget stays default). pending_approval as first-class progress event. Gate: harness + progress_events_emitted (NOT demos unless engine touched).