Add install smoke tests for release artifacts and Homebrew

[andreagrandi]

Summary

Adds credential-free install smoke tests so packaging regressions surface in CI and at release time instead of reaching users. Closes #44.

• scripts/smoke-test.sh — portable smoke test for a logbasset binary: checks --version output, that top-level --help lists every core command, per-command --help, and the no-auth context/schema/completion commands. Exits non-zero on the first failure.
• make smoke-test — builds then runs the script.
• CI (ci.yml) — smoke-tests bin/logbasset after make build on every PR/push, across all Go versions. This is the pre-merge gate.
• Release (release.yml) — extracts the published logbasset_Linux_x86_64.tar.gz and smoke-tests the actual artifact after GoReleaser, catching issues like broken version injection.
• Homebrew — expanded the GoReleaser formula test block from just --version to also cover --help and schema, so brew test logbasset verifies the formula automatically.
• Docs: README "Verifying an Install" section (with the manual Homebrew check), CHANGELOG [Unreleased] entry, AGENTS.md make-target list.

Acceptance criteria

• Release artifacts are smoke-tested automatically (CI + release workflow).
• Homebrew path is verified automatically (brew test) and documented as a manual check.
• Smoke tests require no real API credentials.

Test plan

• make smoke-test passes locally
• Smoke test exits non-zero against a deliberately broken binary
• make test, make vet, gofmt -s -l clean
• CI smoke-test step passes on this PR
• Release workflow smoke-tests the published artifact on the next tag