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/codex/home/config.toml

@@ -0,0 +1,3 @@
+model = "o3"
+
+# Consider setting [mcp_servers] here!

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

@@ -0,0 +1,9 @@
+Attempt to solve the reported issue.
+
+If a code change is required, create a new branch, commit the fix, and open a pull request that resolves the problem.
+
+Here is the original GitHub issue that triggered this run:
+
+### {CODEX_ACTION_ISSUE_TITLE}
+
+{CODEX_ACTION_ISSUE_BODY}

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

@@ -0,0 +1,7 @@
+Review this PR and respond with a very concise final message, formatted in Markdown.
+
+There should be a summary of the changes (1-2 sentences) and a few bullet points if necessary.
+
+Then provide the **review** (1-2 sentences plus bullet points, friendly tone).
+
+{CODEX_ACTION_GITHUB_EVENT_PATH} contains the JSON that triggered this GitHub workflow. It contains the `base` and `head` refs that define this PR. Both refs are available locally.

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

@@ -0,0 +1,7 @@
+Troubleshoot whether the reported issue is valid.
+
+Provide a concise and respectful comment summarizing the findings.
+
+### {CODEX_ACTION_ISSUE_TITLE}
+
+{CODEX_ACTION_ISSUE_BODY}