chore: align XPoster agents/skills and clean up lscache artifacts

[artcava]

Summary

This PR aligns Copilot customizations with the actual XPoster architecture (Azure Functions .NET isolated, timer-triggered workflow), and removes local C# Dev Kit cache artifacts from source control.

Changes included

• Added/updated project-scoped custom agents for XPoster:
  • .NET Functions Architect
  • C# Expert
  • QA
• Added/updated project-scoped skills under .github/skills:
  • azure-functions-isolated-worker
  • azure-functions-timer-trigger
  • csharp-async
  • dotnet-best-practices
  • httpclient-resilience-dotnet
  • xposter-generator-factory
  • xposter-sender-plugin-contract
  • xunit-moq-functions-testing
  • appinsights-functions-observability
  • xposter-config-and-secrets
• Updated .github/copilot-instructions.md to match XPoster runtime and architecture.
• Removed legacy out-of-standard async skill file.
• Added *.lscache to .gitignore.
• Untracked generated C# Dev Kit cache files:
  • src/XPoster.csproj.lscache
  • tests/XPoster.Tests.csproj.lscache

Why

The previous customization set contained patterns from unrelated project templates. This PR makes the guidance deterministic and context-aware for XPoster, reducing wrong suggestions and avoiding accidental commits of local cache artifacts.

Notes

• No runtime feature behavior was intentionally changed.
• Main impact is developer experience and repository hygiene.

[github-actions]

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