Align Desktop 0.5.0 with Core agent onboarding

[askac]

Why

Desktop's Agent menu produces local connection prompts even when the current tab hosts a remote agent. Core 2.12 provides one current-tab onboarding flow for both locations. On Windows, the Core view can also stop receiving input or producing captures after native window activation.

What changed

• Route agent onboarding through Core's Agent Info for Current Tab and Copy Prompt, retaining native Panel, Pause, help and Desktop Core source/recovery controls.
• Disable Windows native occlusion calculation before startup while preserving focus, hidden-window and minimized-window access guards; document the potential background resource cost.
• Pair Desktop 0.5.0 with formal Core 2.12.1, including its Windows executable-path fix, and compare the complete staged and packaged Core payload to that tag.
• Build native Windows x64 and macOS arm64 installers with explicit GUI process completion, first-run runtime preparation/reuse and recorded package identity.

Testing

All 95 Desktop unit tests pass. Native Windows and Windows + WSL source-run tests pass repeated activation/input, Settings focus preservation, floating windows, guarded clipboard operations, PNG capture, decoded WebM recording and hide finalization. The staged Git Core update/recovery integration passes. Native Windows x64 and macOS arm64 packaged application and first-run preparation/reuse checks pass in run 34712625797. The mounted DMG and extracted Windows installer payload match all 78 formal Core files; the extracted Windows application also passes Windows and Windows + WSL input/capture/recording tests.

Packages remain evaluations: real IME and OS clipboard behavior, installer upgrade and Finder/Gatekeeper acceptance require manual checks. Windows is unsigned; macOS uses ad-hoc signing without notarization.