Migrate Mastra agents to file-based directories

Convert agents built with new Agent({...}) and registered in a new Mastra({ agents: {...} }) map into one directory per agent under src/mastra/agents/<name>/. Sibling files replace the old constructor options.

Preserve the agent's behavior. Move each option to the file that owns it, keep the same model, instructions, tools, memory, skills, and subagents, and do not invent config the agent did not have.