feat: add GitLab CI/CD pipeline with .NET 10 SDK

Summary

Adds a complete GitLab CI/CD pipeline that builds, tests, and demonstrates the project using Docker with .NET 10 SDK. Also fixes multiple build errors that prevented .NET 10 compilation.

Changes

Date	Comment/Source	Action Taken
2026-04-09	User request (Telegram)	Created .gitlab-ci.yml with restore, build, test, demo stages; added CHANGELOG.md entry
2026-04-09	Build errors on .NET 10	Fixed switch expression → switch statement; added missing project references; added missing using; fixed undefined variable in demo methods
2026-04-09	Pipeline tag request	Added tags: [docker] to all pipeline jobs

The What

• Integrate GitLab CI/CD for automated build and test
• Ensure project compiles and runs with .NET 10
• Provide demo verification via CLI scenarios

The How

CI/CD Pipeline (.gitlab-ci.yml)

• Uses mcr.microsoft.com/dotnet/sdk:10.0 Docker image
• Stages:
  1. build: dotnet build Release configuration
  2. test: dotnet test with TRX logger and JUnit reports
  3. demo: installs Plannotator and runs all CLI scenarios
• All jobs tagged with docker runner tag
• Demo stage allow_failure: true to avoid blocking pipeline when Plannotator unavailable

Build Fixes (required for .NET 10)

• Switch expression → switch statement: C# 12/.NET 10 no longer allows block bodies in switch expression arms; converted the Main method's scenario dispatcher to a switch statement
• Missing project references: Added <ProjectReference> from CLI and Test projects to the core Protocol library (were missing, causing namespace/type resolution failures)
• Missing using directive: Added using Research.Plannotator.Protocol.Models; to Program.cs for PlannotatorResult
• Undefined variable: Replaced await client.SubmitPlanAsync(plan, origin) with await client.SubmitPlanAsync(plan); origin was undefined and the parameter already has a default value

Validation Plan

1. ✅ Docker image mcr.microsoft.com/dotnet/sdk:10.0 available
2. ✅ dotnet restore and dotnet build succeed locally (verified)
3. ✅ Unit tests compile and run locally (7 tests fail due to pre-existing version comparison logic bug, but build succeeds)
4. ⏳ Merge MR and observe CI pipeline runs successfully

Blast Radius

• No breaking API changes
• Only CI/CD configuration and minor code fixes for .NET 10 compatibility
• No impact on runtime behavior (demo scenarios unchanged)
• Affects only GitLab CI/CD environment