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/workflows/codex.yml

@@ -0,0 +1,76 @@
+name: Codex
+
+on:
+  issues:
+    types: [opened, labeled]
+  pull_request:
+    branches: [main]
+    types: [labeled]
+
+jobs:
+  codex:
+    # This `if` check provides complex filtering logic to avoid running Codex
+    # on every PR. Admittedly, one thing this does not verify is whether the
+    # sender has write access to the repo: that must be done as part of a
+    # runtime step.
+    #
+    # Note the label values should match the ones in the .github/codex/labels
+    # folder.
+    if: |
+      (github.event_name == 'issues' && (
+        (github.event.action == 'labeled' && (github.event.label.name == 'codex-attempt' || github.event.label.name == 'codex-triage'))
+      )) ||
+      (github.event_name == 'pull_request' && github.event.action == 'labeled' && github.event.label.name == 'codex-review')
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write # can push or create branches
+      issues: write # for comments + labels on issues/PRs
+      pull-requests: write # for PR comments/labels
+    steps:
+      # TODO: Consider adding an optional mode (--dry-run?) to actions/codex
+      # that verifies whether Codex should actually be run for this event.
+      # (For example, it may be rejected because the sender does not have
+      # write access to the repo.) The benefit would be two-fold:
+      # 1. As the first step of this job, it gives us a chance to add a reaction
+      #    or comment to the PR/issue ASAP to "ack" the request.
+      # 2. It saves resources by skipping the clone and setup steps below if
+      #    Codex is not going to run.
+
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      # We install the dependencies like we would for an ordinary CI job,
+      # particularly because Codex will not have network access to install
+      # these dependencies.
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+
+      - name: Install dependencies (codex-cli)
+        working-directory: codex-cli
+        run: npm ci
+
+      - uses: dtolnay/rust-toolchain@1.87
+        with:
+          targets: x86_64-unknown-linux-gnu
+          components: clippy
+
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            ${{ github.workspace }}/codex-rs/target/
+          key: cargo-ubuntu-24.04-x86_64-unknown-linux-gnu-${{ hashFiles('**/Cargo.lock') }}
+
+      # Note it is possible that the `verify` step internal to Run Codex will
+      # fail, in which case the work to setup the repo was worthless :(
+      - name: Run Codex
+        uses: ./.github/actions/codex
+        with:
+          openai_api_key: ${{ secrets.CODEX_OPENAI_API_KEY }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          codex_home: ./.github/codex/home