Add Tahoe "Allow in Menu Bar" detection + user guidance dialog

[pdurlej]

Fixes #890.

Summary

• Add a Tahoe-only startup watcher that checks whether visible NSStatusItem buttons actually materialize in a menu bar window/screen.
• Log an error when the status item appears blocked by macOS' new Menu Bar allow-list.
• Show a dismissible guidance dialog with a direct x-apple.systempreferences:com.apple.MenuBarSettings button.
• Persist the guidance state with hasShownTahoeAllowListGuidance, while allowing it to reappear after 24h if the condition persists.
• Cover the detection and re-show policy with focused tests.

Dialog copy

Title: CodexBar can't show its menu bar icon

Body: macOS Tahoe can block menu bar apps in System Settings → Menu Bar → Allow in the Menu Bar. CodexBar is running, but macOS may be hiding its icon. Open Menu Bar settings and turn CodexBar on.

Buttons: Open Menu Bar Settings / Dismiss

Notes

Apple does not expose a public API for the Tahoe allow-list state, so this is intentionally a best-effort heuristic. It only runs immediately after startup to avoid confusing later manual hiding tools such as Bartender with the system-level block.

Root-cause context came from the community diagnosis in #890: #890 (comment)

Testing

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --filter MenuBarVisibilityWatcherTests
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift build
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer make check
• git diff --check

One local note: the default Command Line Tools developer dir failed on KeyboardShortcuts #Preview macros, so the successful build/test/lint runs above use the full Xcode developer dir.