Fix: activate Desktop plan mode only through /plan

[DevMando]

Desktop could unexpectedly switch a normal request into structured plan approval when its complexity heuristic or the model decided to propose a plan. Users now explicitly choose that workflow with /plan; ordinary prompts, including multi-step tasks and prose outlines, run directly.

Desktop sessions enable the shared engine's explicit planning policy, which removes propose_plan from normal tools and updates agent instructions. The controller no longer applies automatic planning heuristics. Plan review, editing, execution, and recovery remain available. Settings explains /plan instead of offering an automatic-planning toggle.

The MandoCode submodule advances from f4c4bd8 to 6915e36 to include the host policy from DevMando/MandoCode#115, along with intervening CLI navigation and Git-review improvements. These CLI interfaces do not replace Desktop's WinUI layout. Changelog updated; local engine launch settings excluded.

Validation

• Desktop application builds successfully for x64 with zero warnings and errors.
• 571/571 Desktop tests pass.
• The pinned engine passed 931/931 tests on .NET 8 and .NET 10, including explicit-planning policy regression coverage.
• Interactive Desktop smoke testing remains to be performed.