Remove zombie tools spec module (#22820)

## 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.

codex-rs/core/src/session/turn_context.rs

@@ -527,7 +527,7 @@ impl Session {
         .with_image_generation_capability(provider_capabilities.image_generation)
         .with_web_search_capability(provider_capabilities.web_search)
         .with_unified_exec_shell_mode_for_session(
-            crate::tools::spec::tool_user_shell_type(user_shell),
+            crate::tools::tool_user_shell_type(user_shell),
             shell_zsh_path,
             main_execve_wrapper_exe,
         )