Migrate Mastra agents to file-based

Convert agents built in code with new Agent({...}) and registered in a new Mastra({ agents: {...} }) map into the file-based convention: one directory per agent under src/mastra/agents/<name>/, where sibling files supply what used to be constructor options.

This is a behavior-preserving refactor, not a redesign. Move each option to the file that owns it, keep the same model, instructions, tools, memory, skills, and subagents, and don't invent config the agent didn't have.

Two things that break migrations

1. File-based agents are discovered ONLY by the Mastra bundler (mastra dev / mastra build). If the app imports the mastra instance directly (Mastra used as a library, a custom server, tests that import { mastra }), agents/<name>/ directories are never discovered and the agent silently disappears. Before migrating, confirm how the app runs: through the CLI → migrate freely; imported directly → do not migrate that agent, keep it in code and tell the user why.