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/env-context.ts

@@ -0,0 +1,116 @@
+/*
+ * Centralised access to environment variables used by the Codex GitHub
+ * Action.
+ *
+ * To enable proper unit-testing we avoid reading from `process.env` at module
+ * initialisation time.  Instead a `EnvContext` object is created (usually from
+ * the real `process.env`) and passed around explicitly or – where that is not
+ * yet practical – imported as the shared `defaultContext` singleton. Tests can
+ * create their own context backed by a stubbed map of variables without having
+ * to mutate global state.
+ */
+
+import { fail } from "./fail";
+import * as github from "@actions/github";
+
+export interface EnvContext {
+  /**
+   * Return the value for a given environment variable or terminate the action
+   * via `fail` if it is missing / empty.
+   */
+  get(name: string): string;
+
+  /**
+   * Attempt to read an environment variable. Returns the value when present;
+   * otherwise returns undefined (does not call `fail`).
+   */
+  tryGet(name: string): string | undefined;
+
+  /**
+   * Attempt to read an environment variable. Returns non-empty string value or
+   * null if unset or empty string.
+   */
+  tryGetNonEmpty(name: string): string | null;
+
+  /**
+   * Return a memoised Octokit instance authenticated via the token resolved
+   * from the provided argument (when defined) or the environment variables
+   * `GITHUB_TOKEN`/`GH_TOKEN`.
+   *
+   * Subsequent calls return the same cached instance to avoid spawning
+   * multiple REST clients within a single action run.
+   */
+  getOctokit(token?: string): ReturnType<typeof github.getOctokit>;
+}
+
+/** Internal helper – *not* exported. */
+function _getRequiredEnv(
+  name: string,
+  env: Record<string, string | undefined>,
+): string | undefined {
+  const value = env[name];
+
+  // Avoid leaking secrets into logs while still logging non-secret variables.
+  if (name.endsWith("KEY") || name.endsWith("TOKEN")) {
+    if (value) {
+      console.log(`value for ${name} was found`);
+    }
+  } else {
+    console.log(`${name}=${value}`);
+  }
+
+  return value;
+}
+
+/** Create a context backed by the supplied environment map (defaults to `process.env`). */
+export function createEnvContext(
+  env: Record<string, string | undefined> = process.env,
+): EnvContext {
+  // Lazily instantiated Octokit client – shared across this context.
+  let cachedOctokit: ReturnType<typeof github.getOctokit> | null = null;
+
+  return {
+    get(name: string): string {
+      const value = _getRequiredEnv(name, env);
+      if (value == null) {
+        fail(`Missing required environment variable: ${name}`);
+      }
+      return value;
+    },
+
+    tryGet(name: string): string | undefined {
+      return _getRequiredEnv(name, env);
+    },
+
+    tryGetNonEmpty(name: string): string | null {
+      const value = _getRequiredEnv(name, env);
+      return value == null || value === "" ? null : value;
+    },
+
+    getOctokit(token?: string) {
+      if (cachedOctokit) {
+        return cachedOctokit;
+      }
+
+      // Determine the token to authenticate with.
+      const githubToken = token ?? env["GITHUB_TOKEN"] ?? env["GH_TOKEN"];
+
+      if (!githubToken) {
+        fail(
+          "Unable to locate a GitHub token. `github_token` should have been set on the action.",
+        );
+      }
+
+      cachedOctokit = github.getOctokit(githubToken!);
+      return cachedOctokit;
+    },
+  };
+}
+
+/**
+ * Shared context built from the actual `process.env`.  Production code that is
+ * not yet refactored to receive a context explicitly may import and use this
+ * singleton.  Tests should avoid the singleton and instead pass their own
+ * context to the functions they exercise.
+ */
+export const defaultContext: EnvContext = createEnvContext();