CI: build/test on macOS and Windows, not just Linux

[stefanthearchitect]

Context

The test and lint jobs run only on ubuntu-latest (.github/workflows/ci.yml). GoReleaser cross-compiles for macOS and Windows at release time, but nothing tests there — so a platform-specific bug (path handling, terminal/TUI behavior, os/exec differences) can ship undetected.

The task

Add macos-latest and windows-latest to the test job via a matrix. Expect to fix a few genuinely platform-specific things (path separators, the inline Bubble Tea renderer, subprocess env) — those fixes are the real value.

Notes / constraints

• The repo's convention is to run tests in Docker locally for host-safety; CI runners are ephemeral VMs where native go test is fine (the workflow already does this on Linux).
• Keep the coverage gate on the Linux leg to avoid triple-counting.

Acceptance

• CI is green on all three OSes (or the platform bugs it surfaces are fixed).
• No flakiness introduced.