Add CLAUDE.md with build-system and swbuild guidance

Document the swift-build architecture, build/test/format workflow, the
swbuild console CLI, and SwiftPM package product static-vs-dynamic linkage
(including the diamond-resolution mechanism and the Xcode 26.5->27.0
regression fixed by PR swiftlang#1424).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: danielpunkass