feat: add greywall update command

[Joyce-O]

Adds greywall update, a new command that updates both greywall and greyproxy to the latest release. The behavior depends on how greywall was installed:

• Installed via install.sh → downloads and replaces the binaries automatically
• Installed via Homebrew → prints the brew upgrade command to run
• Installed from source → prints manual instructions and exits cleanly

--beta flag targets the latest pre-release for both tools. If no beta exists for either tool, a clear error is shown instead of a raw API message.

Both tools are skipped if already on the latest version.

Changes

cmd/greywall/main.go

• greywall update command with install-method detection
• copyFileTo handles atomic binary replacement across filesystems
• Up-to-date check before downloading — skips if already on latest version
• Clear error messages when --beta has no available pre-release

internal/proxy/install.go

• DownloadGreywallBinary fetches asset URL via GitHub API instead of constructing it directly
• resolveGreywallAssetURL resolves the correct greywall release asset for the current platform
• fetchReleaseFor — generic GitHub release fetcher shared across both tools
• CheckLatestTag(beta) / CheckLatestTagFor(owner, repo, beta) — public API for version checking
• Fixed 404 on tagged release fetches — tags/ prefix now applied correctly

README.md

• Added note that go install and source builds don't include greyproxy — run greywall setup separately

Tests

• copyFileTo — atomic replacement across filesystems
• TestCheckLatestTagFor — stable, beta, and error cases via httptest server