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/action.yml

@@ -0,0 +1,124 @@
+name: "Codex [reusable action]"
+description: "A reusable action that runs a Codex model."
+
+inputs:
+  openai_api_key:
+    description: "The value to use as the OPENAI_API_KEY environment variable when running Codex."
+    required: true
+  trigger_phrase:
+    description: "Text to trigger Codex from a PR/issue body or comment."
+    required: false
+    default: ""
+  github_token:
+    description: "Token so Codex can comment on the PR or issue."
+    required: true
+  codex_args:
+    description: "A whitespace-delimited list of arguments to pass to Codex. Due to limitations in YAML, arguments with spaces are not supported. For more complex configurations, use the `codex_home` input."
+    required: false
+    default: "--full-auto"
+  codex_home:
+    description: "Value to use as the CODEX_HOME environment variable when running Codex."
+    required: false
+  codex_release_tag:
+    description: "The release tag of the Codex model to run."
+    required: false
+    default: "codex-rs-d519bd8bbd1e1fd9efdc5d68cf7bebdec0dd0f28-1-rust-v0.0.2505270918"
+
+runs:
+  using: "composite"
+  steps:
+    # Do this in Bash so we do not even bother to install Bun if the sender does
+    # not have write access to the repo.
+    - name: Verify user has write access to the repo.
+      env:
+        GH_TOKEN: ${{ github.token }}
+      shell: bash
+      run: |
+        set -euo pipefail
+
+        PERMISSION=$(gh api \
+          "/repos/${GITHUB_REPOSITORY}/collaborators/${{ github.event.sender.login }}/permission" \
+          | jq -r '.permission')
+
+        if [[ "$PERMISSION" != "admin" && "$PERMISSION" != "write" ]]; then
+          exit 1
+        fi
+
+    - name: Download Codex
+      env:
+        GH_TOKEN: ${{ github.token }}
+      shell: bash
+      run: |
+        set -euo pipefail
+
+        # Determine OS/arch and corresponding Codex artifact name.
+        uname_s=$(uname -s)
+        uname_m=$(uname -m)
+
+        case "$uname_s" in
+          Linux*)   os="linux" ;;
+          Darwin*)  os="apple-darwin" ;;
+          *) echo "Unsupported operating system: $uname_s"; exit 1 ;;
+        esac
+
+        case "$uname_m" in
+          x86_64*) arch="x86_64" ;;
+          arm64*|aarch64*) arch="aarch64" ;;
+          *) echo "Unsupported architecture: $uname_m"; exit 1 ;;
+        esac
+
+        # linux builds differentiate between musl and gnu.
+        if [[ "$os" == "linux" ]]; then
+          if [[ "$arch" == "x86_64" ]]; then
+            triple="${arch}-unknown-linux-musl"
+          else
+            # Only other supported linux build is aarch64 gnu.
+            triple="${arch}-unknown-linux-gnu"
+          fi
+        else
+          # macOS
+          triple="${arch}-apple-darwin"
+        fi
+
+        # Note that if we start baking version numbers into the artifact name,
+        # we will need to update this action.yml file to match.
+        artifact="codex-exec-${triple}.tar.gz"
+
+        gh release download ${{ inputs.codex_release_tag }} --repo openai/codex \
+          --pattern "$artifact" --output - \
+        | tar xzO > /usr/local/bin/codex-exec
+        chmod +x /usr/local/bin/codex-exec
+
+        # Display Codex version to confirm binary integrity; ensure we point it
+        # at the checked-out repository via --cd so that any subsequent commands
+        # use the correct working directory.
+        codex-exec --cd "$GITHUB_WORKSPACE" --version
+
+    - name: Install Bun
+      uses: oven-sh/setup-bun@v2
+      with:
+        bun-version: 1.2.11
+
+    - name: Install dependencies
+      shell: bash
+      run: |
+        cd ${{ github.action_path }}
+        bun install --production
+
+    - name: Run Codex
+      shell: bash
+      run: bun run ${{ github.action_path }}/src/main.ts
+      # Process args plus environment variables often have a max of 128 KiB,
+      # so we should fit within that limit?
+      env:
+        INPUT_CODEX_ARGS: ${{ inputs.codex_args || '' }}
+        INPUT_CODEX_HOME: ${{ inputs.codex_home || ''}}
+        INPUT_TRIGGER_PHRASE: ${{ inputs.trigger_phrase || '' }}
+        OPENAI_API_KEY: ${{ inputs.openai_api_key }}
+        GITHUB_TOKEN: ${{ inputs.github_token }}
+        GITHUB_EVENT_ACTION: ${{ github.event.action || '' }}
+        GITHUB_EVENT_LABEL_NAME: ${{ github.event.label.name || '' }}
+        GITHUB_EVENT_ISSUE_NUMBER: ${{ github.event.issue.number || '' }}
+        GITHUB_EVENT_ISSUE_BODY: ${{ github.event.issue.body || '' }}
+        GITHUB_EVENT_REVIEW_BODY: ${{ github.event.review.body || '' }}
+        GITHUB_EVENT_COMMENT_BODY: ${{ github.event.comment.body || '' }}