Complete the public CLI exit-code contract

[jordancalhoun]

Summary

• document the stable CLI exit-code table and the compatibility change from the historical catch-all status 255
• add dedicated signing failures (status 6) by separating package construction from the productsign stage for both component and distribution packages
• preserve build/process status 5 for non-signing failures and classify stapling with notarization status 7
• cover every documented failure class through the CLI entry point

Closes #54

Verification

• swift test (111 tests)
• ./scripts/verify-loop.sh
• swift build -c release
• xcodebuild -project swiftpkg.xcodeproj -scheme Swiftpkgr -configuration Release CODE_SIGNING_ALLOWED=NO build

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