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/add-reaction.ts

@@ -0,0 +1,85 @@
+import * as github from "@actions/github";
+import type { EnvContext } from "./env-context";
+
+/**
+ * Add an "eyes" reaction to the entity (issue, issue comment, or pull request
+ * review comment) that triggered the current Codex invocation.
+ *
+ * The purpose is to provide immediate feedback to the user – similar to the
+ * *-in-progress label flow – indicating that the bot has acknowledged the
+ * request and is working on it.
+ *
+ * We attempt to add the reaction best suited for the current GitHub event:
+ *
+ *   • issues              → POST /repos/{owner}/{repo}/issues/{issue_number}/reactions
+ *   • issue_comment       → POST /repos/{owner}/{repo}/issues/comments/{comment_id}/reactions
+ *   • pull_request_review_comment → POST /repos/{owner}/{repo}/pulls/comments/{comment_id}/reactions
+ *
+ * If the specific target is unavailable (e.g. unexpected payload shape) we
+ * silently skip instead of failing the whole action because the reaction is
+ * merely cosmetic.
+ */
+export async function addEyesReaction(ctx: EnvContext): Promise<void> {
+  const octokit = ctx.getOctokit();
+  const { owner, repo } = github.context.repo;
+  const eventName = github.context.eventName;
+
+  try {
+    switch (eventName) {
+      case "issue_comment": {
+        const commentId = (github.context.payload as any)?.comment?.id;
+        if (commentId) {
+          await octokit.rest.reactions.createForIssueComment({
+            owner,
+            repo,
+            comment_id: commentId,
+            content: "eyes",
+          });
+          return;
+        }
+        break;
+      }
+      case "pull_request_review_comment": {
+        const commentId = (github.context.payload as any)?.comment?.id;
+        if (commentId) {
+          await octokit.rest.reactions.createForPullRequestReviewComment({
+            owner,
+            repo,
+            comment_id: commentId,
+            content: "eyes",
+          });
+          return;
+        }
+        break;
+      }
+      case "issues": {
+        const issueNumber = github.context.issue.number;
+        if (issueNumber) {
+          await octokit.rest.reactions.createForIssue({
+            owner,
+            repo,
+            issue_number: issueNumber,
+            content: "eyes",
+          });
+          return;
+        }
+        break;
+      }
+      default: {
+        // Fallback: try to react to the issue/PR if we have a number.
+        const issueNumber = github.context.issue.number;
+        if (issueNumber) {
+          await octokit.rest.reactions.createForIssue({
+            owner,
+            repo,
+            issue_number: issueNumber,
+            content: "eyes",
+          });
+        }
+      }
+    }
+  } catch (error) {
+    // Do not fail the action if reaction creation fails – log and continue.
+    console.warn(`Failed to add \"eyes\" reaction: ${error}`);
+  }
+}