Draft changelog: OAuth token exchange broadcast scopes

[mintlify]

Summary

Adds a Sep 23, 2026 changelog entry covering the addition of broadcast.read and broadcast.write to the OAuth 1.0a → OAuth 2.0 token exchange scope mapping.

Changes

• New <Update label="Sep 23, 2026"> entry in changelog.mdx linking to the OAuth 1.0a token exchange page.

Context

Based on PR #448, which was the only product-facing change merged since the previous changelog entry.

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