CI: Customize v8 building (#22086)

## Summary

Move the rusty_v8 artifact production into hermetic Bazel path and bump
the `v8` crate to `147.4.0`

The new flow builds V8 release artifacts from source for Darwin and
Linux targets, publishes both the current release-compatible artifacts
and sandbox-enabled variants, and keeps Cargo consumers on prebuilt
binaries by continuing to feed the `v8` crate the archive and generated
binding files it already expects.

## Why

We need control over V8 build-time features without giving up prebuilt
artifacts for downstream Cargo builds.

Upstream `rusty_v8` already supports source-only features such as
`v8_enable_sandbox`, but its normal prebuilt release assets do not cover
every feature combination we need. Building the artifacts ourselves lets
us enable settings such as the V8 sandbox and pointer compression at
artifact build time, then publish those outputs so ordinary Cargo builds
can still consume prebuilts instead of compiling V8 locally.

This keeps the fast consumer experience of prebuilt `rusty_v8` archives
while giving us a reproducible path to ship featureful variants that
upstream does not currently publish for us.

## Implementation Notes

The Bazel graph in this PR is not copied wholesale from `rusty_v8`;
`rusty_v8`'s normal source build is still GN/Ninja-based.

Instead, this change starts from upstream V8's Bazel rules and adapts
them to Codex's hermetic toolchains and dependency layout. Where we
intentionally follow `rusty_v8`, we mirror its existing artifact
contract:

- the same `v8` crate version and generated binding expectations
- the same sandbox feature relationship, where sandboxing requires
pointer compression
- the same custom libc++ model expected by Cargo's default
`use_custom_libcxx` feature
- the same release-style archive plus `src_binding` outputs consumed by
the `v8` crate

To preserve that contract, the Bazel release path pins the libc++,
libc++abi, and llvm-libc revisions used by `rusty_v8 v147.4.0`, builds
release artifacts with `--config=rusty-v8-upstream-libcxx`, and folds
the matching runtime objects into the final static archive.

## Windows

Windows is annoyingly handled differently.

Codex's current hermetic Bazel Windows C++ platform is `windows-gnullvm`
/ `x86_64-w64-windows-gnu`, while upstream `rusty_v8` publishes Windows
prebuilts for `*-pc-windows-msvc`. Those are different ABIs, so the
Bazel graph cannot truthfully reproduce the upstream MSVC artifacts
until we add a real MSVC-targeting C++ toolchain.

For now:

- Windows MSVC consumers continue to use upstream `rusty_v8` release
archives.
- Windows GNU targets are built in-tree so they link against a matching
GNU ABI.
- The canary workflow separately exercises upstream `rusty_v8` source
builds for MSVC sandbox artifacts, but MSVC is not yet part of the
Bazel-produced release matrix.

## Validation
This PR is technically self validating through CI. I have already
published it as a release tag so the artifacts from this branch are
published to
https://github.com/openai/codex/releases/tag/rusty-v8-v147.4.0 CI for
this PR should therefore consume our own release targets. I have also
locally tested for linux and darwin.

---------

Co-authored-by: Codex <noreply@openai.com>

.codex/skills/update-v8-version/SKILL.md

@@ -0,0 +1,72 @@
+---
+name: update-v8-version
+description: Update Codex's pinned `v8` / `rusty_v8` versions, validate the release-candidate path, and investigate failed V8 canary or artifact builds. Use when asked to bump V8, update `rusty_v8` artifacts, prepare or validate a V8 release candidate, check `v8-canary`, or diagnose why a V8 version update no longer builds.
+---
+
+# Update V8 Version
+
+## Core Workflow
+
+1. Read `third_party/v8/README.md` and follow its version-bump sequence. Treat
+   that document as the release-process source of truth.
+2. Inspect and update the concrete repo surfaces that carry the pin:
+   - `codex-rs/Cargo.toml`
+   - `codex-rs/Cargo.lock`
+   - `MODULE.bazel`
+   - `third_party/v8/BUILD.bazel`
+   - `third_party/v8/README.md`
+   - the matching `third_party/v8/rusty_v8_<version>.sha256` manifest when the
+     remaining prebuilt inputs change
+3. Keep the existing checksum helpers in the loop:
+
+   ```bash
+   python3 .github/scripts/rusty_v8_bazel.py update-module-bazel
+   python3 .github/scripts/rusty_v8_bazel.py check-module-bazel
+   python3 -m unittest discover -s .github/scripts -p test_rusty_v8_bazel.py
+   ```
+
+4. Validate the release-candidate path before broadening the work:
+   - Prefer checking the `v8-canary` CI result for the candidate branch or PR
+     when one exists, using GitHub check tooling or `gh` as appropriate.
+   - If CI is unavailable or the user asked for a local-only check, run the
+     closest local validation that is practical for the changed surface and say
+     explicitly that it is a local substitute, not the full hosted canary.
+5. If the canary path passes, stop there. Summarize the result and encourage the
+   user to commit the candidate changes or proceed with the release flow they
+   requested. Do not publish tags, releases, or pushes unless the user asked.
+
+## Failure Path
+
+Enter this path only when the canary or local build path fails.
+
+1. Capture the failing target, workflow job, and first actionable error.
+2. Compare the currently pinned version with the target version at the relevant
+   upstream tag or SHA. Inspect both:
+   - `denoland/rusty_v8`
+   - upstream V8 source at the target Bazel-pinned version
+3. Track build-relevant deltas rather than broad source churn:
+   - generated binding layout changes
+   - archive or asset naming changes
+   - GN/Bazel target changes
+   - custom libc++ / libc++abi / llvm-libc inputs
+   - sandbox or pointer-compression feature relationships
+   - patch hunks in `patches/` that no longer apply or no longer match upstream
+4. Trace each failing delta back into Codex's build graph:
+   - `MODULE.bazel`
+   - `third_party/v8/BUILD.bazel`
+   - `.github/scripts/rusty_v8_bazel.py`
+   - `.github/workflows/v8-canary.yml`
+   - `.github/workflows/rusty-v8-release.yml`
+5. Update only the pieces required to restore the target version's build and
+   artifact contract. Keep patch explanations and doc changes close to the
+   affected files.
+6. Re-run the focused validation. If it becomes green, return to the normal
+   workflow and stop with a concise summary plus the remaining release step.
+
+## Reporting
+
+- Say whether validation came from hosted `v8-canary` or from a local
+  substitute.
+- Distinguish "version bump complete" from "release published".
+- When blocked, report the upstream delta that matters, the Codex file it hits,
+  and the next concrete fix to try.

.codex/skills/update-v8-version/agents/openai.yaml

@@ -0,0 +1,4 @@
+interface:
+  display_name: "Update V8 Version"
+  short_description: "Guide V8 bumps and release validation"
+  default_prompt: "Use $update-v8-version to update Codex to a new v8 release and validate the release-candidate path."