Extract tool config into codex-tools

[bolinfest]

Why

codex-core already owns too much of the tool stack, and AGENTS.md explicitly pushes us to move shared code out of codex-core instead of letting it keep growing. This PR takes the next incremental step in moving core/src/tools toward codex-rs/tools by extracting low-coupling tool configuration and image-detail gating logic into codex-tools.

That gives later extraction work a cleaner boundary to build on without trying to move the entire tools subtree in one shot.

What changed

• moved ToolsConfig, ToolsConfigParams, shell backend config, and unified-exec session selection from core/src/tools/spec.rs into codex-tools
• moved original image-detail gating and normalization into codex-tools
• updated codex-core to consume the new codex-tools exports and pass a rendered agent-type description instead of raw role config
• kept codex-rs/tools/src/lib.rs exports-only, with extracted unit tests living in sibling *_tests.rs modules

Testing

• cargo test -p codex-tools
• cargo test -p codex-core --lib tools::spec::