Adds "prefer the dedicated tool over bash" guidelines for grep, find, and ls, mirroring the built-in read/write lines. All three ship with guidelines: [], so nothing steers the model away from bash grep/rg, find/fd, and ls. Patches the Guidelines section through the mutable systemPromptOptions of before_agent_start rather than re-registering the tools: re-registering a tool with a built-in name replaces its definition wholesale and would clobber another extension's execute (e.g. ssh's remote-routed grep/find/ls). Experimental local stopgap ahead of proposing the equivalent one-line additions upstream for the built-in tool definitions.