feat(brain): register GET /pass-rate route

Adds the route entry alongside the existing POST routes. Note: this
is the brain HTTP REST API's first GET endpoint — it follows REST
semantics for pure reads, while the legacy POST routes (query, write,
ingest, etc.) all take JSON bodies. Future read endpoints SHOULD use
GET; future write endpoints continue with POST.

ingestion/cmd/server/main.go

@@ -73,6 +73,7 @@ func main() {
 	mux.HandleFunc("POST /ingest-path", h.IngestPath)
 	mux.HandleFunc("POST /ingest-raw", h.IngestRaw)
 	mux.HandleFunc("POST /backfill-refs", h.BackfillRefs)
+	mux.HandleFunc("GET /pass-rate", h.PassRate)
 	mux.Handle("POST /mcp", mcpSrv)
 
 	addr := ":" + port