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/src/comment.ts

@@ -0,0 +1,53 @@
+import type { EnvContext } from "./env-context";
+import { runCodex } from "./run-codex";
+import { postComment } from "./post-comment";
+import { addEyesReaction } from "./add-reaction";
+
+/**
+ * Handle `issue_comment` and `pull_request_review_comment` events once we know
+ * the action is supported.
+ */
+export async function onComment(ctx: EnvContext): Promise<void> {
+  const triggerPhrase = ctx.tryGet("INPUT_TRIGGER_PHRASE");
+  if (!triggerPhrase) {
+    console.warn("Empty trigger phrase: skipping.");
+    return;
+  }
+
+  // Attempt to get the body of the comment from the environment. Depending on
+  // the event type either `GITHUB_EVENT_COMMENT_BODY` (issue & PR comments) or
+  // `GITHUB_EVENT_REVIEW_BODY` (PR reviews) is set.
+  const commentBody =
+    ctx.tryGetNonEmpty("GITHUB_EVENT_COMMENT_BODY") ??
+    ctx.tryGetNonEmpty("GITHUB_EVENT_REVIEW_BODY") ??
+    ctx.tryGetNonEmpty("GITHUB_EVENT_ISSUE_BODY");
+
+  if (!commentBody) {
+    console.warn("Comment body not found in environment: skipping.");
+    return;
+  }
+
+  // Check if the trigger phrase is present.
+  if (!commentBody.includes(triggerPhrase)) {
+    console.log(
+      `Trigger phrase '${triggerPhrase}' not found: nothing to do for this comment.`,
+    );
+    return;
+  }
+
+  // Derive the prompt by removing the trigger phrase. Remove only the first
+  // occurrence to keep any additional occurrences that might be meaningful.
+  const prompt = commentBody.replace(triggerPhrase, "").trim();
+
+  if (prompt.length === 0) {
+    console.warn("Prompt is empty after removing trigger phrase: skipping");
+    return;
+  }
+
+  // Provide immediate feedback that we are working on the request.
+  await addEyesReaction(ctx);
+
+  // Run Codex and post the response as a new comment.
+  const lastMessage = await runCodex(prompt, ctx);
+  await postComment(lastMessage, ctx);
+}