fix: clarify full board data tool description

[omribz156]

Summary

• replace the scary INTERNAL USE ONLY full-board-data tool description with clearer guidance
• keep the steer toward narrower board tools for normal conversational requests
• add a focused test so the warning does not come back

Verification

• npx prettier@3.2.4 --check packages/agent-toolkit/src/core/tools/platform-api-tools/full-board-data-tool/full-board-data-tool.ts packages/agent-toolkit/src/core/tools/platform-api-tools/full-board-data-tool/full-board-data-tool.test.ts --ignore-path .prettierignore
• git diff --check

I also attempted the focused Jest test via npx, but this fresh clone has no workspace dependencies installed and the TypeScript Jest config could not resolve @jest/types from the temporary npx install. I avoided doing a full workspace install for this small copy/test change.

This was implemented with Codex assistance, with the final diff manually reviewed and kept focused.

Closes #366.