## Summary
- mark `ToolSearch` as removed and ignore stale config writes for its
legacy key
- make search tool exposure depend only on model capability, not a
feature toggle
- remove app-server enablement support and prune now-obsolete test
coverage/setup
## Verification
- `cargo test -p codex-features`
- `cargo test -p codex-tools`
- `cargo test -p codex-core search_tool_requires_model_capability`
- `cargo test -p codex-app-server experimental_feature_enablement_set_`
## Notes
- This keeps the legacy config key as a no-op for compatibility while
removing the ability to toggle the behavior off cleanly.
- No developer-facing docs update outside the touched app-server README
was needed.