Timing Ph4: aggregate summary event #220

Emit one aggregate timing summary per run: total, setup, planning, execution, synthesis, llm, mcp, overhead.

State as of 2026-07-17: nothing like this exists. The closest artifact is the run manifest's phase_timings (persistence.rs:115), which covers the final iteration only and is not exposed as an SSE event. Depends on the #218 measurements existing before an aggregate can be honest about the overhead bucket.