CI and release automation via GitHub Actions

ci.yml: make test on every push/PR, Linux only - matches the README's
own honest claim that Linux is the only platform actually run and
verified, rather than having CI pretend untested platforms are
behaviorally covered. A separate cross-build job runs make release on
the same Linux runner as a build-only check across every platform in
the Makefile's matrix (CGO_ENABLED=0 + pure-Go SQLite means no
macOS/Windows runner is needed just to catch a compile break).

release.yml: fires on a v* tag, runs make release, packages each
platform into a single archive (.tar.gz Unix, .zip Windows) plus a
SHA256SUMS file, and attaches them to a GitHub Release via the gh CLI
rather than a third-party action - matches this project's own general
preference for minimizing external dependencies.

Verified the packaging shell logic locally (not the YAML itself, which
needs a real Actions run) against a real make release output: all 6
platform archives build with correct internal structure, zip selected
only for Windows, SHA256SUMS covers all of them.

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