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