Implement agent definition loader (typespec and JSON)

[bgervin]

PRD: Agent Definition Loader

Objective

Build a module to load agent definitions from typespec or JSON. If typespec is provided, compile to JSON first. Support local and GitHub sources.

Tasks

• Integrate typespec-to-JSON compilation pipeline.
• Implement local file and GitHub file fetching (support auth for private repos).
• Parse JSON using Microsoft.DeclarativeAgents.Manifest Nuget library.
• Expose API for downstream modules to access agent definition.

Acceptance Criteria

• Loader accepts both typespec and JSON.
• Can fetch files from disk or GitHub.
• Returns parsed agent definition object.