ci: add GitHub Actions workflow with test matrix, lint, and coverage

I added `.github/workflows/ci.yml` with three jobs:

1. test — Emacs 27.1 / 28.2 / 29.4 / 30.1, runs `make compile` (strict
   warnings) and `make test-all`. fail-fast off so one version's failure
   doesn't hide others.
2. lint (advisory) — `eask lint package`, `eask lint checkdoc`, and `make
   lint` (elisp-lint). All three are `continue-on-error: true` because there's
   an existing MELPA-prep backlog (1 package-lint error in chime-org-contacts.el,
   ~17 cosmetic checkdoc/package-lint warnings) that's worth surfacing without
   blocking CI. Tighten to required once the backlog is cleared.
3. coverage — runs the full suite with undercover and uploads to Coveralls
   via the official action. No secret needed because the repo is public —
   GITHUB_TOKEN is enough.

Two supporting changes:

- `tests/run-coverage-file.el` now switches between simplecov (local) and
  coveralls (CI, detected via the `CI` env var GitHub Actions sets
  automatically) report formats. The Coveralls action expects coveralls JSON.
- `Makefile`'s `coverage' target now runs ALL_TESTS with selector `t', not
  UNIT_TESTS with `(not (tag :slow))'. Without this the integration tests
  contributed nothing to the reported coverage number.

1 files changed, 100 insertions, 0 deletions

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..6ad5618
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,100 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: Test (Emacs ${{ matrix.emacs_version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        emacs_version: ['27.1', '28.2', '29.4', '30.1']
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Emacs
+        uses: jcs090218/setup-emacs@master
+        with:
+          version: ${{ matrix.emacs_version }}
+
+      - name: Set up Eask
+        uses: emacs-eask/setup-eask@master
+        with:
+          version: 'snapshot'
+
+      - name: Install dependencies
+        run: eask install-deps --dev
+
+      - name: Byte-compile (warnings are errors)
+        run: make compile
+
+      - name: Run full test suite
+        run: make test-all
+
+  lint:
+    name: Lint (advisory)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Emacs
+        uses: jcs090218/setup-emacs@master
+        with:
+          version: '29.4'
+
+      - name: Set up Eask
+        uses: emacs-eask/setup-eask@master
+        with:
+          version: 'snapshot'
+
+      - name: Install dependencies
+        run: eask install-deps --dev
+
+      # Lint output is informational while the MELPA-prep backlog is being
+      # worked through. Tighten to required once that backlog is clear.
+      - name: package-lint
+        continue-on-error: true
+        run: eask lint package
+
+      - name: checkdoc
+        continue-on-error: true
+        run: eask lint checkdoc
+
+      - name: elisp-lint
+        continue-on-error: true
+        run: make lint
+
+  coverage:
+    name: Coverage
+    runs-on: ubuntu-latest
+    needs: test
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Emacs
+        uses: jcs090218/setup-emacs@master
+        with:
+          version: '29.4'
+
+      - name: Set up Eask
+        uses: emacs-eask/setup-eask@master
+        with:
+          version: 'snapshot'
+
+      - name: Install dependencies
+        run: eask install-deps --dev
+
+      - name: Generate coverage report
+        run: make coverage
+
+      - name: Upload to Coveralls
+        uses: coverallsapp/github-action@v2
+        with:
+          file: .coverage/coveralls.json
+          format: coveralls