Add GitHub custom agent for MIDL language server development

[Copilot]

Adds a GitHub custom agent to assist with development and maintenance of the MIDL 3.0 language server VSCode extension.

Changes

• Agent configuration (.github/agents/midl-dev.agent.md)
  • YAML frontmatter: full tool access, expert persona in LSP and VSCode extension development
  • Explicit prompt to reference MIDL 3 documentation for syntax/semantics
  • Project architecture: client/server LSP implementation, PEG.js parser, TypeScript build chain
  • Development workflows: build, test, debug (F5 launch), parser modification, feature implementation
  • Common tasks: parser fixes, diagnostics, language features, CI/CD
  • Priority order: security → parser bugs → diagnostics → features → quality → docs

Agent Capabilities

The agent understands:

• MIDL 3.0 language constructs (runtime classes, interfaces, attributes, namespaces)
• Language Server Protocol implementation patterns
• VSCode Extension API and semantic token providers
• PEG.js grammar debugging and modification
• TypeScript/Node.js ecosystem for language servers

Documentation references updated to learn.microsoft.com URLs.

Original prompt

create a github agent for this repo to aid in development and maintenance. instructions about custom agents: https://gh.io/customagents/config
the repo is the home for the midl-langserv project which is a VSCode extension (vsix) for MIDL 3 support. MIDL 3 is the Microsoft IDL v3 language, which is an interface definition language that allows defining WinRT APIs.

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