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/main.ts

@@ -0,0 +1,80 @@
+#!/usr/bin/env bun
+
+import type { Config } from "./config";
+
+import { defaultContext, EnvContext } from "./env-context";
+import { loadConfig } from "./load-config";
+import { setGitHubActionsUser } from "./git-user";
+import { onLabeled } from "./process-label";
+import { ensureBaseAndHeadCommitsForPRAreAvailable } from "./prompt-template";
+import { performAdditionalValidation } from "./verify-inputs";
+import { onComment } from "./comment";
+import { onReview } from "./review";
+
+async function main(): Promise<void> {
+  const ctx: EnvContext = defaultContext;
+
+  // Build the configuration dynamically by scanning `.github/codex/labels`.
+  const GITHUB_WORKSPACE = ctx.get("GITHUB_WORKSPACE");
+  const config: Config = loadConfig(GITHUB_WORKSPACE);
+
+  // Optionally perform additional validation of prompt template files.
+  performAdditionalValidation(config, GITHUB_WORKSPACE);
+
+  const GITHUB_EVENT_NAME = ctx.get("GITHUB_EVENT_NAME");
+  const GITHUB_EVENT_ACTION = ctx.get("GITHUB_EVENT_ACTION");
+
+  // Set user.name and user.email to a bot before Codex runs, just in case it
+  // creates a commit.
+  setGitHubActionsUser();
+
+  switch (GITHUB_EVENT_NAME) {
+    case "issues": {
+      if (GITHUB_EVENT_ACTION === "labeled") {
+        await onLabeled(config, ctx);
+        return;
+      } else if (GITHUB_EVENT_ACTION === "opened") {
+        await onComment(ctx);
+        return;
+      }
+      break;
+    }
+    case "issue_comment": {
+      if (GITHUB_EVENT_ACTION === "created") {
+        await onComment(ctx);
+        return;
+      }
+      break;
+    }
+    case "pull_request": {
+      if (GITHUB_EVENT_ACTION === "labeled") {
+        await ensureBaseAndHeadCommitsForPRAreAvailable(ctx);
+        await onLabeled(config, ctx);
+        return;
+      }
+      break;
+    }
+    case "pull_request_review": {
+      await ensureBaseAndHeadCommitsForPRAreAvailable(ctx);
+      if (GITHUB_EVENT_ACTION === "submitted") {
+        await onReview(ctx);
+        return;
+      }
+      break;
+    }
+    case "pull_request_review_comment": {
+      await ensureBaseAndHeadCommitsForPRAreAvailable(ctx);
+      if (GITHUB_EVENT_ACTION === "created") {
+        await onComment(ctx);
+        return;
+      }
+      break;
+    }
+  }
+
+  console.warn(
+    `Unsupported action '${GITHUB_EVENT_ACTION}' for event '${GITHUB_EVENT_NAME}'.`,
+  );
+}
+
+main();