**Disabling Image-Gen for Non-SIWC Codex Users**
We are only enabling image-gen feature for SIWC Codex users until there
comes a fix in ResponsesAPI to omit output from responses.completed, to
prevent the following issues:
1. websocket blows up due to heavier load (images) than before (text)
2. http parser streams through n^2 of n-base64 bytes (sum of base64s of
all images generated in turn) that causes long delays in
turn_completion.
## 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::`
Michael Bolin