Document dynamic tool definition patterns

[1aifanatic]

Summary

• document callback indirection when only a tool's execution behavior changes
• document unregister-and-register replacement for agent-visible metadata changes
• explain toolchange refresh behavior and execution-time revalidation
• keep update APIs, lazy schemas, and disabled/grouped states explicitly in their existing design discussions

Verification

• git diff --check

Fixes #167.

[domfarolino]

I pushed some nits and rewording changes up. I think this looks good. I'm glad we're documenting the discussions surrounding dynamic tools, in #167 and #255 for future reference. Since #167 motivated this and we're not planning on adding APIs to update model-facing bits of a tool just yet, I think we can close that issue out for now and re-open it if we see strong use cases for such an API in the future.