fix(core): clarify code mode tool boundary - #38785

Summary

• move the invariant tool-availability boundary into the execute tool description
• tell agents to ignore tools outside the Code Mode catalog when writing execute code
• keep changing catalog instructions focused on catalog and search contents

Testing

• bun test test/codemode/catalog.test.ts test/codemode/instructions.test.ts test/tool-execute.test.ts
• bun typecheck
• git diff --check