execpolicy2 core integration (#6641)

This PR threads execpolicy2 into codex-core.

activated via feature flag: exec_policy (on by default)

reads and parses all .codexpolicy files in `codex_home/codex`

refactored tool runtime API to integrate execpolicy logic

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/Cargo.lock

@@ -1086,6 +1086,7 @@ dependencies = [
  "codex-apply-patch",
  "codex-arg0",
  "codex-async-utils",
+ "codex-execpolicy2",
  "codex-file-search",
  "codex-git",
  "codex-keyring-store",