Nightly dev builds for internal testing

[blsmth]

GoReleaser already runs on every push to main in --snapshot mode, producing binaries that are never published. We should make these available for internal testing so Pete, Brendan, and Darwin can test the latest main without cutting a public release.

Options to explore:

• Upload snapshot binaries as GitHub Actions artifacts (already possible — just need to add an actions/upload-artifact step after GoReleaser)
• Publish to a private/pre-release GitHub Release (e.g. nightly tag, overwritten each night)
• Push to a separate Homebrew tap (e.g. apppackio/homebrew-dev)

Requirements:

• Available via a simple install/download — no git clone && go build
• Does not affect the public release channel (Homebrew tap, GitHub Releases)
• Ideally works on macOS (arm64 + amd64) and Linux

Simplest path: Add actions/upload-artifact to the existing goreleaser workflow for non-tag pushes. Artifacts are downloadable from the Actions UI for 90 days.

[blsmth]

Turns out snapshot binaries are already uploaded as artifacts on every push — per-platform (darwin amd64/arm64, linux amd64, windows amd64). They're downloadable from the Actions tab on any workflow run.

What's missing is discoverability — you have to know to go to Actions → goreleaser → pick a run → download artifacts. Options to improve:

1. Add retention-days to the existing uploads (currently unlimited, could trim to 14 days)
2. Add a "latest snapshot" workflow that creates/updates a pre-release GitHub Release tagged nightly — gives a stable download URL
3. Just document the current artifact download flow for Pete and Darwin

Closing as the core functionality already exists. Can reopen if we want the nightly release tag approach.