improve: reduce Codex history preparation imports (#154404)

improve: reduce Codex history preparation imports (#154404)

* improve: reduce Codex history preparation imports

* test: isolate historical Node runner policy from CI

The sparse workflow-owner fixture inherited the new bun-compatible host
policy and attempted to import runtime discovery that its five-file Node
adapter intentionally excludes. Own both the fixture's Node policy and
its empty job environment overlay before running the real boundary.

Reproduced all three failures from CI35560122489 on main. The unchanged
six cases pass after this repair; 102 owner and runner sibling tests pass
with conflicting inherited policy. All selected checks and independent
review pass. No production code, timeout, retry, or assertions changed.