feat: upgrade to .NET 9 / C# 13, latest packages & actions, add Copilot instructions (#2)

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for HihaArvio
+
+## Authoritative Reference
+
+**CLAUDE.md is the single source of truth for this repository.**
+
+All guidelines, architecture decisions, implementation requirements, and coding standards defined in [`CLAUDE.md`](../CLAUDE.md) at the repository root MUST be followed without exception. CLAUDE.md takes absolute precedence over any other instruction, convention, or assumption.
+
+Before suggesting, generating, or reviewing any code for this repository, read and internalize CLAUDE.md in its entirety.
+
+## Non-Negotiable Rules
+
+The following rules are derived from CLAUDE.md and additional Copilot-specific code quality guidelines. All MUST be enforced in every interaction:
+
+### Language & Framework
+
+- **Target framework:** .NET 9, using C# 13
+- **UI framework:** .NET MAUI (multi-platform)
+- **Nullable reference types MUST be enabled** across all projects
+- **All compiler warnings MUST be treated as errors** (`<TreatWarningsAsErrors>true</TreatWarningsAsErrors>`)
+- Language version MUST be set to `13` in all project files
+
+### Architecture (MVVM – Strict Separation)
+
+- **Models**: Plain data objects only — no business logic
+- **ViewModels**: All presentation logic — 100% testable without UI dependencies
+- **Views**: Thin layer — data binding only, minimal code-behind
+- **Services**: All business logic and infrastructure concerns
+
+### Dependency Injection
+
+- All services MUST be injected via constructor — never use the service locator pattern or `new` inside ViewModels
+- Register all services in `MauiProgram.cs`
+- Platform-specific implementations use `#if IOS`, `#elif WINDOWS || MACCATALYST`
+
+### Testing Requirements
+
+- Test coverage MUST be ≥ 95 % (measured by Coverlet; threshold enforcement is a future CI/CD enhancement)
+- Testing stack: **xUnit + NSubstitute + FluentAssertions**
+- All tests MUST use deterministic, seeded randomness — never `Random.Shared`
+- All external dependencies (sensors, database, file system) MUST be mocked
+- Use test-data builders for complex objects
+
+### Security
+
+- Estimate selection MUST use `System.Security.Cryptography.RandomNumberGenerator` — never `System.Random`
+- No external data transmission — all data is stored locally only
+- Request the minimum required platform permissions
+
+### Commit & Code Quality
+
+- Use **semantic / conventional commit** messages (e.g. `feat:`, `fix:`, `chore:`, `docs:`, `test:`, `refactor:`)
+- Make **atomic commits** — one logical change per commit
+- Run all linting and build tools; automatically fix all reported issues before committing
+- Follow the `.editorconfig` rules present in the repository root
+
+## Technology Stack (Current Versions)
+
+| Component | Version |
+|---|---|
+| .NET SDK | 9.0.x |
+| C# Language | 13 |
+| Microsoft.Maui.Controls | 9.0.120 |
+| Microsoft.Maui.Controls.Compatibility | 9.0.120 |
+| CommunityToolkit.Mvvm | 8.4.0 |
+| SQLite (sqlite-net-pcl) | 1.9.172 |
+| SQLitePCLRaw.bundle_green | 2.1.11 |
+| Microsoft.Extensions.Logging.Debug | 9.0.13 |
+| xUnit | 2.9.3 |
+| xunit.runner.visualstudio | 2.8.2 |
+| Microsoft.NET.Test.Sdk | 18.3.0 |
+| NSubstitute | 5.3.0 |
+| FluentAssertions | 8.8.0 |
+| coverlet.collector | 8.0.0 |
+
+## Key Files
+
+| File | Purpose |
+|---|---|
+| `CLAUDE.md` | **Authoritative project guide** — always read first |
+| `spec.md` | Formal RFC 2119 requirements (MUST / SHALL / REQUIRED) |
+| `docs/plans/2025-11-18-hiha-arvio-design.md` | Validated architecture decisions |
+| `src/HihaArvio/HihaArvio.csproj` | Main MAUI project |
+| `tests/HihaArvio.Tests/HihaArvio.Tests.csproj` | Unit & integration tests |
+| `.editorconfig` | Code style enforcement |
+
+## Easter Egg — Never Expose
+
+The hidden humorous mode triggered by shaking for > 15 seconds MUST NOT have any visible UI indication and MUST NOT be mentioned in any UI element, setting, or documentation visible to users.
+
+## When in Doubt
+
+If there is any ambiguity, consult **CLAUDE.md** and **spec.md** — in that order.  
+Do not invent architecture, add dependencies, or change platform targets without explicit justification grounded in those documents.