feat: weight merged PRs above LOC in usage leaderboard sorting (#1563)

Move merged_prs to the primary sort key in the agent usage leaderboard,
ahead of agent_loc, prs_opened, and agent_runs. Update the UI description
to reflect the new ranking order.

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

agent/dashboard/agent_usage.py

@@ -398,6 +398,7 @@ async def _build_usage_leaderboard_snapshot(period: Period) -> dict[str, Any]:
     sorted_users = sorted(
         users.values(),
         key=lambda item: (
+            -item["merged_prs"],
             -item["agent_loc"],
             -item["prs_opened"],
             -item["agent_runs"],

ui/src/routes/usage.tsx

@@ -63,7 +63,7 @@ function UsagePage() {
     <AppShell user={session.data} title="Usage" className="max-w-5xl">
       <SettingsSection
         title="Agent leaderboard"
-        description="Ranked by agent lines of code, then PRs opened and agent runs."
+        description="Ranked by merged PRs, then agent lines of code, PRs opened, and agent runs."
         action={
           <Select
             value={activePeriod}