Update CLAUDE.md with mcs export command and Export/ architecture

[bguidolim]

Problem

PR #178 added mcs export and three new source files under Sources/mcs/Export/, but CLAUDE.md was not updated. Every other command and source directory is documented there.

Missing sections

1. ## Commands code block — add mcs export with its flags (--global, --non-interactive, --dry-run, --identifier)
2. ### Commands (Sources/mcs/Commands/) — add ExportCommand.swift
3. New subsection ### Export (Sources/mcs/Export/) — document ConfigurationDiscovery.swift, ManifestBuilder.swift, PackWriter.swift

From PR #178 review.