Clean up macOS menu activity abstractions

[basnijholt]

Summary

• move menu activity priority/timing state into a dedicated MenuActivityTracker
• replace duplicated submenu activity item/separator fields with MenuActivityStatusRow
• add Swift coverage for tracker priority and command restoration behavior

Verification

• swift build --package-path macos/AgentCLI
• uv run pytest -q tests/test_macos_app.py
• uv run pre-commit run --all-files
• git diff --check

Note: local swift test --package-path macos/AgentCLI --enable-xctest links the test bundle but cannot execute here because this Command Line Tools install fails xcrun --sdk macosx --show-sdk-platform-path. CI macOS runners should execute the XCTest target.