Fix duplicate warnings for ATC220 and ATC221

[davidkallesen]

Summary

When both ATC220 and ATC221 are enabled (both default to enabled), a single using directive like using System; produces two warnings — one from each rule. This PR makes the rules complementary by having ATC220 automatically exclude namespaces that ATC221 handles, so each using directive triggers at most one warning.

A new .editorconfig option dotnet_diagnostic.ATC220.exclude_common_namespaces (default: true) controls this behavior. ATC220 reads ATC221's namespace_prefixes to determine which namespaces to skip, keeping the two rules in sync even with custom prefix configurations.

Changes

✨ Features

• Add exclude_common_namespaces option to ATC220 (default: true) that defers System/Microsoft/Atc namespaces to ATC221
• ATC220 reads ATC221's namespace_prefixes configuration to stay in sync with custom prefixes
• Add GetExcludeCommonNamespaces helper to AnalyzerConfigOptionsExtensions

♻️ Refactoring

• Update sample apps to remove unnecessary #pragma warning disable directives now that rules don't overlap
• Update existing ATC220 tests to explicitly set exclude_common_namespaces = false where testing core "flag everything" behavior
• Add new tests for default exclusion behavior and opt-out scenario

📝 Documentation

• Replace "recommended to enable only one" guidance with "Working Together" section in ATC220.md and ATC221.md
• Document the new exclude_common_namespaces configuration option with behavior matrix
• Update CLAUDE.md with automatic mutual exclusivity details

Notes

• Setting exclude_common_namespaces = false restores the original ATC220 behavior (flags all namespaces)
• If ATC221 is disabled and you want ATC220 to cover everything, set exclude_common_namespaces = false in .editorconfig