chore(cli): add changelog entry for x-fern-playground to x-fern-explorer rename

[devin-ai-integration]

Add changelog entry for x-fern-playground to x-fern-explorer rename

Summary

Adds a new changelog entry for CLI version 0.120.0 documenting the breaking change where OpenAPI extensions x-fern-playground and x-fern-playground-enabled have been renamed to x-fern-explorer and x-fern-explorer-enabled respectively.

This is a companion PR to the implementation PR in fern-api/fern: fern-api/fern#10537

Review & Testing Checklist for Human

• Verify the changelog entry accurately describes the breaking change and matches the implementation in the fern repo PR
• Confirm this PR should be merged in coordination with the fern repo PR (ideally around the same time to keep docs in sync)
• Check if any other documentation pages need updating (e.g., OpenAPI extension reference documentation, if it exists)

Notes

• This is a breaking change (type: break) because the old extension keys are no longer recognized
• The changelog follows the existing format: version number, type tag, and description
• Historical changelog entries (2025-11-12.mdx and 2025-11-05.mdx) were intentionally left unchanged to preserve the historical record

Requested by: Deep Singhvi ([email protected]) (@dsinghvi)
Devin session: https://app.devin.ai/sessions/1c4aa288493b41c6ae8f8d251f8f88f6

[devin-ai-integration]

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