Phase 00: Dead code deletion and test hygiene

[knausj85]

Executes Phase 00 of the architecture modernization spec.

Changes

• Named-pipe surface deleted (per external control surface decision #33): NamedPipeService.cs (including the NamedPipeCommand/CommandType types declared in it), NamedPipeClient/HintOverlayClient.cs, Examples/HintOverlayClientExamples.cs, the commented-out DI registration and controller wiring, and the pipe-only OnNamedPipeCommandReceived/SelectHintByLabel handlers (the latter had no other callers). The README''s keystroke-based Talon integration is untouched.
• Orphaned test references removed (per testing decision #36): xunit.assert/xunit.v3.extensibility.core package references in the app csproj (no xunit usage exists in the project) and the CI "Clean Debug output" step that scrubbed xunit.* from the Debug artifact.
• Spec correction: the phase doc claimed NamedPipeCommand/CommandType lived in Models.cs; they were declared at the bottom of NamedPipeService.cs.

Definition of done

• Solution builds (Debug, full MSBuild path) with no references to the deleted types — verified locally; Debug output contains no xunit.* files
• Straggler sweep: no NamedPipe/HintOverlayClient/CommandType references remain outside the spec docs
• CI green (lint + Debug + Release jobs)
• Manual smoke test sections 1, 4, 6 (hint workflow, taskbar mode, tray/lifecycle)

No behavior change.

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