docs(prompt): tighten shell tool usage guidance wording

[tusharmath]

Summary

Tighten the wording in the shell tool's usage guidance to use clearer, more directive language that better communicates intent to the model.

Context

The shell tool description contained slightly ambiguous phrasing in two guidance notes. "You do NOT need to use" implies it's optional, while the actual intent is a firm prohibition. Similarly, "Avoid using" is weaker than the intended directive. Stronger, unambiguous language reduces the chance the model will fall back to shell-based file operations when dedicated tools should be used instead.

Changes

• Changed "you do NOT need to use head, tail, or other truncation commands" → "you should NOT use" to make the prohibition explicit
• Changed "Avoid using shell with find, grep, cat..." → "Do not use" for consistent, directive tone across all guidance notes

Testing

Snapshot tests updated to reflect the new wording in the rendered tool descriptions.

cargo insta test --accept