chore: reuse open sync PR instead of opening a new one each run

[muhsinking]

Summary

• Switches the docs-sync workflow to a fixed sync-openapi branch and force-pushes each run, so consecutive openapi.yaml changes update the existing PR in mintlify-docs instead of stacking new ones.
• Only calls gh pr create when no open PR exists for the branch; otherwise logs that the existing PR was updated.
• Adds muhsinking to the assignee list for newly created sync PRs.

Notes / tradeoffs

• Force-pushes to the sync-openapi branch — fine since it's bot-only, but any hand-edits to that branch would be overwritten.
• If a sync PR is closed without merging, the next run will open a fresh PR (we only check --state open). This seemed like the right default — closing signals "don't want this" — but easy to change if we'd rather reopen.
• Each sync replaces the previous diff rather than adding commits, so the PR always shows "current main → latest spec" instead of a running history of syncs.

Test plan

• Merge, then push a no-op change to openapi.yaml and confirm a single PR opens in mintlify-docs.
• Push a second change before merging the first and confirm the existing PR gets updated (no second PR).
• Merge or close the PR, push another change, and confirm a fresh PR is created.

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