chore: Update to .NET 10 and refactor extensions

[sirkirby]

• Changed target framework from net9.0 to net10.0 across multiple files, including project files and test configurations.
• Using new c-sharp 14 language features
• Refactored storage directory resolution methods to use the new EffectiveStorageDirectory property instead of the GetEffectiveStorageDirectory method.
• Updated Windows notification provider comments to reflect the new target framework.
• Adjusted integration and unit tests to align with the new framework and method changes.
• Updated all agent files to using .net 10 and csharp 14 moving forward

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This pull request upgrades the project from .NET 9 and C# 12+ to .NET 10 and C# 14 across all documentation, configuration, and build scripts. The update ensures consistency in the target framework and language features throughout the codebase and supporting materials.

Project-wide upgrade to .NET 10 and C# 14:

• Updated all references in documentation files (README.md, AGENTS.md, CLAUDE.md, .github/copilot-instructions.md, .specify/memory/constitution.md, specs/001-os-notifications/WINDOWS-IMPLEMENTATION.md) to specify .NET 10 and C# 14, including required features and technology stack descriptions. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Build, CI, and tooling updates:

• Changed all build, CI, and launch configurations to target .NET 10 instead of .NET 9, including the Makefile, GitHub Actions workflows (pr-validation.yml, release.yml), and VS Code tasks and launch settings. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Feature set and dependency updates:

• Updated documentation to reflect new required C# 14 features (such as extension members and field-backed properties) and clarified dependency requirements for .NET 10. [1] [2]

This upgrade ensures the project leverages the latest language and runtime features, and that all developer tooling and documentation are aligned with the new .NET 10 baseline.