Update AGENTS.md for view-model architecture

[danielchalmers]

What

Updates the agent guidance to match the architecture established in #34 and #35, so future agents extend the codebase the right way instead of re-growing a god-file.

Removed

• "Preserve existing MVVM patterns … unless asked to refactor." — when the existing pattern was a 996-line Window that was also its own view-model, this line actively told agents to keep piling on. (The useful half — bindings/XAML style — is already covered under the XAML section.)

Added

• Architecture section documenting the two projects: SentryReplay.Data (pure domain/playback logic, no WPF/Flyleaf — keep it that way) and the WPF app (views + view-models + services), plus the rule that view-models hold no control references.
• Where new code goes section: new feature logic/state/commands go in a view-model, not window code-behind; don't default to the biggest existing file; keep code-behind small (flag *.xaml.cs over ~150 lines or holding non-view logic).
• Testing guidance now asks for tests on new view-model/domain logic and notes view-models are directly constructible in tests (MainWindowViewModelTests).

Why

Docs-only. The earlier guidance was the main thing steering agents toward the god-file; the structural work in #34/#35 only sticks if the written guidance reinforces it.

Notes

Final step of the reorg plan. Deliberately still no Data→Domain rename and no per-feature slice folders. The VideoPlayerController split (829 lines) remains a deferred, separate effort.