## Why
`list_dir` still carries a full spec/handler/test path, but nothing in
the current model catalog advertises it via
`experimental_supported_tools`. That leaves us maintaining an
environment-backed tool surface that is effectively unused.
## What changed
- delete the `list_dir` handler and its tests from `codex-core`
- remove the `list_dir` spec builder, handler kind, and registry wiring
from `codex-tools`
- clean up the remaining internal README and registry tests so they no
longer mention the removed tool
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