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.

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