feat: initial import of experimental GitHub Action

[bolinfest]

This is a first cut at a GitHub Action that lets you define prompt templates in .md files under .github/codex/labels that will run Codex with the associated prompt when the label is added to a GitHub pull request.

For example, this PR includes these files:

.github/codex/labels/codex-attempt.md
.github/codex/labels/codex-review.md
.github/codex/labels/codex-triage.md

And the new .github/workflows/codex.yml workflow declares the following triggers:

on:
  issues:
    types: [opened, labeled]
  pull_request:
    branches: [main]
    types: [labeled]

as well as the following expression to gate the action:

jobs:
  codex:
    if: |
      (github.event_name == 'issues' && (
        (github.event.action == 'labeled' && (github.event.label.name == 'codex-attempt' || github.event.label.name == 'codex-investigate-issue'))
      )) ||
      (github.event_name == 'pull_request' && github.event.action == 'labeled' && github.event.label.name == 'codex-code-review')

Note the "actor" who added the label must have write access to the repo for the action to take effect.

After adding a label, the action will "ack" the request by replacing the original label (e.g., codex-review) with an -in-progress suffix (e.g., codex-review-in-progress). When it is finished, it will swap the -in-progress label with a -completed one (e.g., codex-review-completed).

Users of the action are responsible for providing an OPENAI_API_KEY and making it available as a secret to the action.