Fix action install on GitHub runners and document Ruby prerequisites

.changes/unreleased/Fixed-20261108-215904.yaml

@@ -1,3 +1,3 @@
 kind: Fixed
-body: Fix `gem install` failing with `BadRequirementError` at action refs with no `version` input, which installs the action's version or latest release.
+body: Fix `gem install` failing with `BadRequirementError` at action refs with no `version` input.
 time: 2026-11-08T21:59:04.551561+01:00

.changes/unreleased/Fixed-20261108-222426.yaml

@@ -0,0 +1,3 @@
+kind: Fixed
+body: 'Fix `se: command not found` failing the action''s lint step on GitHub-hosted runners.'
+time: 2026-11-08T22:24:26.846058+01:00

.github/workflows/ci.yml

@@ -143,6 +143,20 @@ jobs:
             echo "native=false" >> "$GITHUB_OUTPUT"
           fi
 
+  dogfood:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      # The action's install step has no unit-testable seam: it only
+      # exists on a real runner. Run the repo's own action at HEAD, with
+      # no version input, so a local ref installs the latest released
+      # gem and the lint step exercises the PATH wiring end to end.
+      - uses: ./
+        with:
+          files: README.md
+
   rules:
     runs-on: ubuntu-24.04
     timeout-minutes: 10

README.md

@@ -72,6 +72,27 @@ The full list, with a wrong and a right example for each rule [docs/RULES.md](do
 
 ## Install
 
+### Prerequisites
+
+The gem needs Ruby 3.3 or newer. On a Mac, the system Ruby (2.6) is too
+old. Install one, for example with Homebrew:
+
+```bash
+brew install ruby
+```
+
+Homebrew's Ruby is not on the `PATH` by default, and neither are gem
+binaries. Put both in your shell profile:
+
+```bash
+echo 'export PATH="/opt/homebrew/opt/ruby/bin:$PATH"' >> ~/.zshrc
+echo 'export GEM_HOME="$HOME/.gem"' >> ~/.zshrc
+echo 'export PATH="$GEM_HOME/bin:$PATH"' >> ~/.zshrc
+```
+
+Then open a new terminal. A version manager such as `mise` or `rbenv`
+works too.
+
 ### Ruby gem (macOS arm64, Linux x86-64 and arm64)
 
 ```bash

action.yml

@@ -23,6 +23,10 @@ runs:
       shell: bash
       env:
         ACTION_REF: ${{ github.action_ref }}
+        # GitHub-hosted runners make the system gem dir unwritable, so gem
+        # falls back to a user dir the runner never puts on PATH. Pin a home
+        # we control and put its bin on PATH for the Lint step below.
+        GEM_HOME: ${{ runner.temp }}/gems
       run: |
         VERSION="${{ inputs.version }}"
         if [ -z "$VERSION" ]; then
@@ -35,6 +39,10 @@ runs:
         else
           gem install simple_english --no-document
         fi
+        echo "$GEM_HOME/bin" >> "$GITHUB_PATH"
+        # The se bin stub resolves the gem through GEM_HOME, so the home
+        # must reach the Lint step, not just the bin dir.
+        echo "GEM_HOME=$GEM_HOME" >> "$GITHUB_ENV"
     - name: Lint
       id: lint
       shell: bash