Agent: macOS support — ship osx binary + Homebrew tap so `brew install` works

[ChrisonSimtian]

Problem

There is no way to install the agent on macOS. brew install is impossible today — and unlike winget, the gap starts at step zero: no Mac binary is even built.

Current state

• ❌ No osx-arm64 / osx-x64 RID in the publish-agent matrix in .github/workflows/release-images.yml (only win-x64 + linux-x64), so no macOS artifact is attached to releases.
• ❌ No Homebrew formula/cask anywhere in the repo, and no homebrew-tap repo under the ChrisonSimtian org.
• ⚠️ src/Agent/INSTALL.md states macOS isn't supported by --install in v1 (no launchd integration); the binary runs cross-platform but has no service/auto-start path.

Acceptance criteria

• Add osx-arm64 (and likely osx-x64) to the publish-agent matrix; attach erp-agent-osx-arm64.tar.gz (+ x64) to each GitHub Release.
• Create a ChrisonSimtian/homebrew-tap repo with a formula for erp-agent pointing at the release tarball + SHA256.
• Optionally wire a CI job (mirror of publish-winget) to bump the formula version/SHA on each tag.
• Document the brew tap ChrisonSimtian/tap && brew install erp-agent flow in INSTALL.md.
• Verify on Apple Silicon end-to-end.

Out of scope (track separately if wanted)

• launchd service registration for erp-agent --install on macOS (the auto-start-at-login story). This issue is just about installability via brew; the binary can still be run manually per current INSTALL.md.