feat: initial import of experimental GitHub Action (#1170)

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-code-review.md
.github/codex/labels/codex-investigate-issue.md
```

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

```yaml
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.

.github/actions/codex/package.json

@@ -0,0 +1,21 @@
+{
+    "name": "codex-action",
+    "version": "0.0.0",
+    "private": true,
+    "scripts": {
+        "format": "prettier --check src",
+        "format:fix": "prettier --write src",
+        "test": "bun test",
+        "typecheck": "tsc"
+    },
+    "dependencies": {
+        "@actions/core": "^1.11.1",
+        "@actions/github": "^6.0.1"
+    },
+    "devDependencies": {
+        "@types/bun": "^1.2.11",
+        "@types/node": "^22.15.21",
+        "prettier": "^3.5.3",
+        "typescript": "^5.8.3"
+    }
+}