## Summary
- make the shared `ToolExecutor::is_mutating` default conservative by
returning `true`
- update the trait docs to say read-only tools should opt out explicitly
- add a regression test covering the default behavior
## Why
Hosts use this signal for serialization and approval policy. Treating
unknown contributed tools as read-only lets a write-capable tool
accidentally bypass mutating-tool safeguards if it forgets to override
the hook.
## Validation
- not run, per request
## Why
The tool-extraction work needs one shared executable-tool seam that
hosts and tool owners can depend on without reaching into `codex-core`.
Landing that seam first makes the later tool-family ports incremental
and keeps the reusable contract separate from any one migration.
## What changed
- add a new `codex-tool-api` crate and workspace wiring
- move the common executable-tool contracts into that crate:
`ToolBundle`, `ToolDefinition`, `ToolExecutor`, `ToolCall`, `ToolInput`,
`ToolOutput`, `JsonToolOutput`, and `ToolError`
- keep host state generic through `ToolBundle<C>` / `ToolCall<C>` so
later integrations can provide their own runtime context without baking
core types into the API
- carry the host signals the runtime will need later, including
parallel-call support and mutability probing
- leave existing tool families in place for now; this PR only
establishes the reusable API surface
- add the Bazel target and lockfile updates for the new crate
## Testing
- `cargo test -p codex-tool-api`
