fix: complete Desktop engine migration and sync merged updates

[DevMando]

Desktop's RazorConsole migration branch never reached main, and its engine reference still pointed to an earlier maintenance commit. This PR brings the outstanding migration changes into main and updates Desktop to the merged engine revision that passed the Windows/Linux CI matrix.

What changes

• Bundle engine commit 9e25af4, including the MandoCode 0.16/RazorConsole 0.6 migration, code maintenance work, and CI fixes from CLI #133 and #134.
• Include the existing migration-branch improvements: explicit /plan activation, clearer messages for retired Ollama models, and the snapshot-type compatibility fix.
• Consolidate outdated alpha and intermediate engine entries in the unreleased changelog so it describes the actual revision Desktop now uses.

Desktop continues to use its existing WinUI conversation interface. This PR includes the outstanding work from razorconsole-v0.6.0-migration; a separate migration-to-main PR is unnecessary once this PR is merged.

Validation

• Desktop build succeeded with zero warnings/errors against the updated engine.
• All 571 Desktop tests passed.
• The pinned engine passed all 1,244 tests on both .NET 10 and .NET 8, plus all four Windows/Linux CI jobs and repeated component checks in CLI #134.
• Manual Desktop checks for model switching, snapshots/history, multiple agents, approvals, and formatting remain a pre-merge check.

Related: DevMando/MandoCode#133, DevMando/MandoCode#134, and Desktop #82/#83.