Add nightly releases via GoReleaser and Homebrew tap

[jcleira]

Description

Add a nightly release workflow so users and contributors can test pre-release builds of Partio without waiting for a stable release. This mirrors how entireio/cli ships nightly builds (entireio/cli#825).

What to implement

1. .github/workflows/nightly.yml — a cron-driven workflow that checks for new commits since the last nightly tag, computes the next nightly tag (v<patch>-nightly.YYYYMMDD), and triggers GoReleaser.

2. GoReleaser nightly config — mark the release as pre-release, upload binaries for Linux/macOS/Windows, publish to GitHub Releases.

3. Homebrew tap — add a partio-nightly formula so users can install with brew install partio-io/tap/partio-nightly.

4. Version string — embed the nightly tag so partio version clearly indicates a pre-release build.

Why

Pre-release builds let users test bug fixes and new agents before a stable release. This is valuable for a CLI deeply integrated into Git workflows.

Source

Inspired by entireio/cli#825

Acceptance Criteria

• A .github/workflows/nightly.yml workflow runs on daily cron and creates nightly tags only when new commits exist
• GoReleaser builds nightly binaries for Linux, macOS (amd64 + arm64), and Windows
• Homebrew tap formula for partio-nightly is updated
• partio version output identifies nightly builds
• Stable release workflow is not affected