fix(ci): skip upstream CLA assistant on forks

[VibeSWE]

The inherited CLA Assistant workflow fails on this fork because it targets the signature store in parseablehq/.github and the required PERSONAL_ACCESS_TOKEN is unavailable. Add a job condition so CLA enforcement runs only in parseablehq/parseable; jobs on forks are skipped before the action accesses the upstream signature store.

Validation: parsed .github/workflows/cla.yaml with Ruby's YAML parser, confirmed the condition on CLAAssistant, and ran git diff --check successfully.

The workflow uses pull_request_target, so this PR can still run the existing base-branch workflow and show the same failure until the change is merged into main. After merging, trigger a new CLA workflow run for existing PRs (for example, by commenting recheck) to apply the condition.

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