fix(#2840): exclude runtime from defaults.json spread into project config (#2985)

* test(#2840): add regression for runtime poisoning from defaults.json

* fix(#2840): exclude runtime from defaults.json spread into project config

runtime is host-specific (written by whichever installer ran last). On a
machine with 2+ runtimes, it poisons every new project config — e.g. a Codex
install's runtime:'codex' leaks into Claude Code projects. Now excluded from
the userDefaults spread, mirroring the resolve_model_ids guard (#2297).

* chore(#2840): add changeset fragment

* fix(#2840): add new test file to lint-test-file-count allowlist

* chore(#2840): backfill changeset PR number 2985

---------

Co-authored-by: sim <sim@local>

scripts/lint-test-file-count.allowlist.json

@@ -3,6 +3,7 @@
   "modules": {
     "config": {
       "files": [
+        "config-defaults-runtime-exclusion.test.cjs",
         "config-field-docs.test.cjs",
         "config-get-default.test.cjs",
         "config-schema.property.test.cjs",