refactor: extract main window context model

[teamleaderleo]

Summary

Move the nested AppDelegate.MainWindowContext model into its own top-level CmuxMainWindowContext file. Keep the nested typealias so existing extensions, tests, and call sites remain source-compatible while the model gains an independent ownership boundary.

Validation

• git diff --check
• xcrun swiftc -frontend -parse across the context, registry, AppDelegate, Dock extension, and test support files
• scripts/check-pbxproj.sh

Full Xcode build is currently blocked because this worktree does not contain vendor/bonsplit/Package.swift.

[teamleaderleo]

Consolidated into the coherent upstream slices #78 (UI-test infrastructure), #79 (session lifecycle), #80 (window ownership), and #82 (terminal diagnostics). The original branch remains available for history.