Allowlist internal author account in CLA workflow

[Copilot]

The CLAAssistant Actions job was failing on an internal PR because the commit author resolved to slerner-bitdrift, which was not covered by the workflow allowlist. This updates the CLA workflow to treat that internal account the same way as the existing trusted bot/internal identities.

• Root cause

  • contributor-assistant evaluates PR commit authors, not just the commenter who triggers recheck
  • the failing PR’s commits resolved to slerner-bitdrift, so the workflow continued to report an unsigned contributor

• Change

  • extend .github/workflows/cla.yaml allowlist with slerner-bitdrift

• Scope

  • workflow-only change
  • no behavior change for external contributors

allowlist: 'bitdrift-opening-capture-sdk-prs[bot],Copilot,slerner-bitdrift'

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• CHANGELOG.md's "Unreleased" section has been updated, if applicable.