#350: Remove deprecated 'mcs sync --update' flag

[bguidolim]

Closes #350

Summary

mcs sync --update was soft-deprecated one release ago (it warned and pointed at mcs update). It's fully superseded by mcs update, which fetches and re-applies across all configured scopes and — unlike --update — respects the opt-in generate-lockfile config instead of force-writing the lockfile. Now that the deprecation window has passed, this removes it so the CLI carries no deprecated surfaces and no divergent fetch-then-apply code path.

Changes

• mcs sync --update is gone — invoking it now errors with Unknown option '--update'. Users who relied on it should switch to mcs update (pair with mcs config set generate-lockfile true if they want a lockfile written).

Test plan

• swift test passes locally
• swiftformat --lint . and swiftlint pass without violations
• Run mcs sync --update → expect an Unknown option '--update' error
• Run mcs update → fetches latest pack versions and re-applies across scopes
• Run mcs sync with generate-lockfile: true → still writes mcs.lock.yaml

Checklist for engine changes

• Docs updated if behavior changed (CLAUDE.md, docs/, techpack.yaml schema in ExternalPackManifest.swift)