agent simulation: merge accuracy_score and experience_score into overall_score (#1800)

* agent simulation: merge accuracy_score and experience_score into overall_score

Both composites were weighted means over disjoint member sets with no
consumer comparing one against the other. One weighted mean over the
union of members replaces them on JobMetrics and RunMetrics; the old
field numbers and names are reserved.

* generated protobuf

---------

Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: u9g

.changeset/simulation-overall-score.md

@@ -0,0 +1,6 @@
+---
+"github.com/livekit/protocol": patch
+"@livekit/protocol": patch
+---
+
+agent simulation: merge `accuracy_score` and `experience_score` into a single `overall_score` on `SimulationRun.JobMetrics` and `SimulationRun.RunMetrics`; the old field numbers and names are reserved.