## Summary
- move tool_user_shell_type out of the old tools::spec module and call
it from tools directly
- attach the remaining spec planning model tests under spec_plan
- delete core/src/tools/spec.rs
## Tests
- just fmt
- cargo test -p codex-core tools::spec_plan
Note: a broader cargo test -p codex-core run on the earlier PR-head
worktree still hit the pre-existing stack overflow in
agent::control::tests::spawn_agent_fork_last_n_turns_keeps_only_recent_turns.