refactor: remove Python sidecar from C# projects
- Delete PythonSidecarAiService, PythonSidecarSpeechService, PythonSidecarClient
- Remove PythonExecutable, PythonAiSidecarPath, AiSidecarTimeoutMs from JournalConfig
- Remove python-sidecar as valid AiProvider (only none/llamasharp remain)
- Simplify DI: default IAiService is DisabledAiService, ISpeechBridgeService is disabled
- Remove python-sidecar fallback from Journal.AI ServiceCollectionExtensions
- Remove 5 Python sidecar smoke tests and BuildAiConfig helper
- Remove Python config assertions from TestConfigServiceParityKeysAsync
Co-Authored-By: Oz <oz-agent@warp.dev>
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