core: add tool-free thread mode

[river-oai]

Summary

• Add an opt-in tool_free feature for lightweight helper threads.
• Prevent MCP startup and refresh work for tool-free sessions.
• Skip skill, plugin, and tool enumeration, then enforce an empty tool router.

Why

Helper turns such as thread-title generation should not start unrelated MCP servers or expose tools they cannot use. A slow server can otherwise consume the helper's full timeout.

Validation

• codex-features: 55 tests passed.
• MCP refresh/runtime: 2 focused tests passed.
• Tool planning: 28 tests passed.
• cargo check -p codex-app-server passed.
• Scoped Clippy and formatting passed.
• Runtime control: the normal path started the configured MCP server and sent 11 tools; tool_free started no MCP server and sent zero tools.

argument-comment-lint is currently blocked because its pinned Rust 1.92 nightly cannot compile SQLx 0.9, which requires Rust 1.94.

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