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/load-config.ts

@@ -0,0 +1,56 @@
+import type { Config, LabelConfig } from "./config";
+
+import { getDefaultConfig } from "./default-label-config";
+import { readFileSync, readdirSync, statSync } from "fs";
+import * as path from "path";
+
+/**
+ * Build an in-memory configuration object by scanning the repository for
+ * Markdown templates located in `.github/codex/labels`.
+ *
+ * Each `*.md` file in that directory represents a label that can trigger the
+ * Codex GitHub Action. The filename **without** the extension is interpreted
+ * as the label name, e.g. `codex-review.md` ➜ `codex-review`.
+ *
+ * For every such label we derive the corresponding `doneLabel` by appending
+ * the suffix `-completed`.
+ */
+export function loadConfig(workspace: string): Config {
+  const labelsDir = path.join(workspace, ".github", "codex", "labels");
+
+  let entries: string[];
+  try {
+    entries = readdirSync(labelsDir);
+  } catch {
+    // If the directory is missing, return the default configuration.
+    return getDefaultConfig();
+  }
+
+  const labels: Record<string, LabelConfig> = {};
+
+  for (const entry of entries) {
+    if (!entry.endsWith(".md")) {
+      continue;
+    }
+
+    const fullPath = path.join(labelsDir, entry);
+
+    if (!statSync(fullPath).isFile()) {
+      continue;
+    }
+
+    const labelName = entry.slice(0, -3); // trim ".md"
+
+    labels[labelName] = new FileLabelConfig(fullPath);
+  }
+
+  return { labels };
+}
+
+class FileLabelConfig implements LabelConfig {
+  constructor(private readonly promptPath: string) {}
+
+  getPromptTemplate(): string {
+    return readFileSync(this.promptPath, "utf8");
+  }
+}