## Why
Code mode must link against a V8 build with sandbox support, but Windows MSVC
still used upstream non-sandboxed prebuilts and package builds selected the
older release artifact profile.
## What changed
- Enable the `v8_enable_sandbox` feature directly for the code mode runtime.
- Select the `ptrcomp_sandbox_release` archive and bindings in Cargo packaging
and CI, including Windows MSVC release builds.
- Point Bazel's Windows MSVC targets at the sandbox-enabled Codex artifacts and
enable the matching crate feature.
## Testing
Add a runtime test that calls `v8__V8__IsSandboxEnabled()` and verifies that
the linked V8 library has sandbox support enabled.
GitOrigin-RevId: c1b49b44a6ccfea5b5006d69ec7866848d1cddd7
## What changed
- Move the V8 implementation into a dedicated `codex-code-mode-runtime` crate used by `codex-code-mode-host`, removing the embedded runtime fallback from the Codex process.
- Resolve the host executable from the active installation layout and check its availability before selecting tools.
- Fall back to direct tools with a one-time warning when optional code mode is unavailable. Keep `code_mode_only` and `disable_in_process_fallback` configurations fail-closed.
## Testing
- Cover host discovery for standalone and package layouts, including missing hosts and symlinks.
- Verify direct-tool fallback, one-time warnings, and fail-closed code-mode-only behavior.
GitOrigin-RevId: 5aa3c6f1db148b2231fc24089a2ee0e2b00dbddb
