Ability to request codebase_search tool via @ menu as @codebase

[hi5hi5]

Add codebase_search tool to @ menu as @codebase or similar

[elianiva]

i suggested similar idea back then, but make it more general so we don't have to hardcode everything
basically expose all the tools kinda like how github copilot does it
in copilot, @ is reserved for tools and # is for context mention