Proposal: add ignorefile for AI-agents

Bundles would require the way to store some temporal data, which should be ignored by AI-agents. There could be many cases, some of what I can think right now are:
* cache directory to store download progress
* build directory for files produced by HTML consumer
* bundle as a git submodule with .git directory inside

While system files could be stored outside of the bundle, it would bring more technical debt and tools would need to decide where to store this files anyway, this is what can make the process of managing such knowledge bases cumbersome. It's easier to put everything related to the bundle inside of the bundle's own dir

So it would be useful to add ignorefile to tell AI-agents to ignore some files or directories. The name is a thing to consider, the first which I can imagine is `.agentignore`.