Add install smoke tests for go install, release artifacts, and Homebrew formula

[andreagrandi]

Closes #19.

This PR adds automated smoke tests for the documented install paths and release artifacts, plus release verification documentation.

Changes

• Add .github/workflows/install-smoke-tests.yml with three jobs:
  • go-install: builds via go install ./cmd/mb-cli and verifies --help and version.
  • release-artifacts: runs GoReleaser in snapshot mode, executes the Linux amd64 binary, and verifies all expected archives and checksums exist.
  • homebrew-formula: generates the Homebrew formula and syntax-checks it with ruby -c.
• Add scripts/install-smoke-tests.sh for local runs and wire it to make install-smoke-tests.
• Add docs/RELEASE.md with automated smoke test summary and manual post-release verification steps for go install, Homebrew, and binary downloads.
• Update AGENTS.md release process to reference docs/RELEASE.md.
• Update CHANGELOG.md.

Verification

• make test and make vet pass.
• actionlint passes on all workflow files.
• shellcheck passes on the smoke test script.
• make install-smoke-tests passes locally.