Remove zombie tools spec module

[jif-oai]

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.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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