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, 9 insertions, 3 deletions

diff --git a/tests/run-coverage-file.el b/tests/run-coverage-file.el
index 0a07696..d96f8d2 100644
--- a/tests/run-coverage-file.el
+++ b/tests/run-coverage-file.el
@@ -31,13 +31,19 @@
 ;; otherwise overwrite the value.
 (setq undercover-force-coverage t)
 
+;; Local runs emit simplecov for whatever local tooling wants it.  CI sets
+;; CI=true (GitHub Actions does this automatically), so we emit a coveralls
+;; JSON instead and leave it on disk for the upload action to pick up.
 ;; The `undercover' macro splices each configuration list into `(list ,@it)',
 ;; which evaluates the elements.  Wildcard strings have to stay atoms — using
 ;; `(:files ...)' form lets us evaluate `expand-file-name' to an absolute path.
 (undercover (:files (expand-file-name "chime.el" run-coverage--project-root))
-            (:report-format 'simplecov)
-            (:report-file (expand-file-name ".coverage/simplecov.json"
-                                            run-coverage--project-root))
+            (:report-format (if (getenv "CI") 'coveralls 'simplecov))
+            (:report-file (expand-file-name
+                           (if (getenv "CI")
+                               ".coverage/coveralls.json"
+                             ".coverage/simplecov.json")
+                           run-coverage--project-root))
             (:merge-report t)
             (:send-report nil))