[ci] Tighten import-time margin to 15%

CI measured 126.2ms against 123ms baseline + 25% ceiling (153.8ms). That's
27.6ms of headroom — enough slack that a real regression could hide under
it. Drop to 15% (ceiling 141.5ms, ~15ms headroom over observed). Still
well above GHA runner variance, tight enough to catch the class of
regression we care about (zeroconf-style top-level import additions).

script/import_time_budget.json

@@ -1,5 +1,5 @@
 {
   "target_module": "esphome.__main__",
-  "margin_pct": 25,
+  "margin_pct": 15,
   "cumulative_us": 123000
 }