Clarify js_repl binding reuse guidance

[fjord-oai]

Summary

Clarify the js_repl prompt guidance around persistent bindings and redeclaration recovery.

This updates the generated js_repl instructions in core/src/project_doc.rs to prefer this order when a name is already bound:

1. Reuse the existing binding
2. Reassign a previously declared let
3. Pick a new descriptive name
4. Use { ... } only for short-lived scratch scope
5. Reset the kernel only when a clean state is actually needed

The prompt now also explicitly warns against wrapping an entire cell in block scope when the goal is to reuse names across later cells.

Why

The previous wording still left too much room for low-value workarounds like whole-cell block wrapping. In downstream browser rollouts, that pattern was adding tokens and preventing useful state reuse across js_repl cells.

This change makes the preferred behavior more explicit without changing runtime semantics.

Scope

• Prompt/documentation change only
• No runtime behavior changes
• Updates the matching string-backed project_doc tests