[Story]: Set custom instructions per filetype/model mux

[danbarr]

Description

Once per-filetype muxing is introduced into workspaces, it would be interesting to allow custom instructions at that level, in addition to or instead of instructions at the workspace level.

For example, docs/writing-specific instructions for *.md, language for framework-specific instructions for *.js vs. *.py etc.

Could be especially useful in monorepos or multi-language/multi-technology projects.

Additional Context

/cc @aponcedeleonch, based on our quick brainstorm today