`codex-core` had accumulated config loading, requirements parsing,
constraint logic, and config-layer state handling in a single crate.
This change extracts that subsystem into `codex-config` to reduce
`codex-core` rebuild/test surface area and isolate future config work.
## What Changed
### Added `codex-config`
- Added new workspace crate `codex-rs/config` (`codex-config`).
- Added workspace/build wiring in:
- `codex-rs/Cargo.toml`
- `codex-rs/config/Cargo.toml`
- `codex-rs/config/BUILD.bazel`
- Updated lockfiles (`codex-rs/Cargo.lock`, `MODULE.bazel.lock`).
- Added `codex-core` -> `codex-config` dependency in
`codex-rs/core/Cargo.toml`.
### Moved config internals from `core` into `config`
Moved modules to `codex-rs/config/src/`:
- `core/src/config/constraint.rs` -> `config/src/constraint.rs`
- `core/src/config_loader/cloud_requirements.rs` ->
`config/src/cloud_requirements.rs`
- `core/src/config_loader/config_requirements.rs` ->
`config/src/config_requirements.rs`
- `core/src/config_loader/fingerprint.rs` -> `config/src/fingerprint.rs`
- `core/src/config_loader/merge.rs` -> `config/src/merge.rs`
- `core/src/config_loader/overrides.rs` -> `config/src/overrides.rs`
- `core/src/config_loader/requirements_exec_policy.rs` ->
`config/src/requirements_exec_policy.rs`
- `core/src/config_loader/state.rs` -> `config/src/state.rs`
`codex-config` now re-exports this surface from `config/src/lib.rs` at
the crate top level.
### Updated `core` to consume/re-export `codex-config`
- `core/src/config_loader/mod.rs` now imports/re-exports config-loader
types/functions from top-level `codex_config::*`.
- Local moved modules were removed from `core/src/config_loader/`.
- `core/src/config/mod.rs` now re-exports constraint types from
`codex_config`.
