[dotnet-ai] MEAI chat integration

[luisquintanilla]

Fixes #227
Part of #225

Dependencies

Merge order: This PR should be merged after #237 Plugin scaffold.

Summary

Adds the meai-chat-integration skill the foundation layer that most other dotnet-ai skills build upon. Guides developers through integrating LLM chat capabilities using Microsoft.Extensions.AI and the IChatClient abstraction.

Also includes references/tokenizers.md, a comprehensive reference for Microsoft.ML.Tokenizers covering all 8 tokenizer types (Tiktoken, BPE, SentencePiece, Llama, BERT, WordPiece, CodeGen, Phi2) and core operations (CountTokens, EncodeToIds, Decode, GetIndexByTokenCount).

Changes

File	Description
plugins/dotnet-ai/skills/meai-chat-integration/SKILL.md	IChatClient setup, ChatClientBuilder middleware pipeline, streaming, structured output, retry/resilience, model pinning, token counting, output validation
plugins/dotnet-ai/skills/meai-chat-integration/references/tokenizers.md	Microsoft.ML.Tokenizers API reference token counting, truncation, encoding/decoding, cross-skill usage table
tests/dotnet-ai/meai-chat-integration/eval.yaml	Eval: ASP.NET Core chat endpoint scenario

Key Packages

• Microsoft.Extensions.AI
• Microsoft.ML.Tokenizers
• Provider: OpenAI, Azure.AI.OpenAI, or OllamaSharp

Validation

• SKILL.md has valid YAML frontmatter (name, description)
• Covers IChatClient setup, streaming, structured output, middleware, resilience
• references/tokenizers.md covers all 8 tokenizer types and core operations
• eval.yaml has at least one scenario
• Does not duplicate content from other skills

[luisquintanilla]

Closing this skill's content has been bundled into #237 (scaffold + meai-chat-integration) per reviewer feedback that a blank plugin definition should not merge without content. Issue #227 is now addressed by #237.