[codex] Add package release workflow

[jayscambler]

Summary

This adds a dedicated GitHub Actions release workflow for publishing the Python and npm packages through the existing release environment.

Today the repo has CI coverage and a configured release environment, but no workflow that actually builds, validates, and publishes package artifacts. That leaves package publication as a manual local process even though the repository already has release-environment approval configured.

What This Adds

The new workflow is manual (workflow_dispatch) and supports releasing from a chosen ref. It validates artifacts before any publish step runs, then gates the actual publish jobs behind environment: release so the existing reviewer approval remains the final control point.

The workflow does the following:

• builds Python sdist and wheel from autocontext/
• smoke-installs the built Python wheel and runs autoctx --help
• builds the npm package from ts/
• packs the npm tarball and verifies key contents like README.md and dist/index.js
• uploads the validated artifacts between jobs
• publishes to PyPI using PYPI_API_TOKEN
• publishes to npm using NPM_TOKEN

Why This Matters

This closes the gap between having release credentials/environment protections configured and actually being able to use them. It also makes package publication reproducible and reviewable instead of relying on an ad hoc local release flow.

Validation

I validated the workflow structure locally and also separately validated the release artifacts from main:

• Python 0.2.1 builds successfully with uv build
• the Python wheel installs into a fresh virtualenv and autoctx --help runs
• the npm package builds successfully
• the npm tarball includes README.md and built dist/ output