Add FindMRForBranch helper to check for existing MR beads before creating.
If an MR already exists for the branch, skip creation and reuse it.
This makes gt done safe to re-run if interrupted mid-execution.
Implements Option C from gt-svdsy: idempotent operations that check
if already done before doing, making it safe to retry.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
c2a33be4e6