feat: source-based install and greywall update command

[Joyce-O]

feat: source-based install and greywall update command

Replaces the pre-built binary download with a source build, and adds a greywall update command to keep both tools up to date.
greywall setup

• Now builds greyproxy from source (git clone + go build + greyproxy install --force) instead of downloading a release asset — works on Linux and any architecture without a published binary

greywall update

• New command: fetches the latest stable tags for both greyproxy and greywall, rebuilds each from source
• --beta flag to target pre-release tags
• updateSelf — clones greywall at the target tag, runs make build-ci, atomically replaces the running binary (falls back to copy if cross-filesystem rename fails)

Internal

• Added CheckLatestTag / CheckLatestTagFor for GitHub API tag resolution (stable and pre-release)
• Replaced InstallOptions with SourceBuildOptions (Tag, Beta, Output)
• Removed dead Install and helpers (resolveAssetURL, downloadAsset, extractTarGz)

[Joyce-O]

Moved update-cmd and fix-setup-and-docs