docs: mention tool name migration

[Kage18]

Summary

Fixes #12213.

Adds a short AI SDK 6 migration guide section explaining that tool({ name }) was removed and that the tool name now comes from the key in the tools object.

Tests

• git diff --check

[Kage18]

Kage usage for this PR:

This PR fixed #12213 by documenting the AI SDK 6 tool-name migration: tool({ name }) was removed and the tool name now comes from the key in the tools object.

How Kage helped:

• Scoped recall to the migration-guide docs area instead of searching unrelated provider/package code.
• Identified the right target doc: content/docs/08-migration-guides/24-migration-guide-6-0.mdx.
• Kept the change intentionally docs-only and aligned with the existing migration-guide style.
• Captured the reusable migration note so future agents can recall the rule when users ask why tool({ name }) no longer works.
• Captured the lightweight verification path: git diff --check.

Why this matters for future sessions:
A future agent handling tool migration questions can recall the exact behavioral rule and documentation location instead of re-reading the migration guide or guessing from old API examples.

Full contribution summary and Kage metrics: kage-core/Kage#8

[lgrammel]

Thank you for the pull request. Issue #12213 was closed as completed. Closing this pull request.