Migrate Claude actions to Codex

[jrandolf-old]

Summary

• Replace the Claude Code release-note step with the pinned Codex GitHub Action.
• Replace the Claude review and mention workflows with read-only Codex workflows.
• Switch the mention trigger from @claude to @codex and post Codex output back to the issue or pull request.
• Use OPENAI_API_KEY and remove the no-longer-needed OIDC permission.

Validation

• actionlint on all changed workflows
• YAML parse and git diff --check

Required setup

Add the OPENAI_API_KEY organization secret before merging; it is not currently configured.

[jrandolf-old]

Staying on Claude Code actions for now; closing this migration.