Add git helper functions for merge request conflict checking:
- FetchBranch(remote, branch): fetch a specific branch from remote
- CheckConflicts(source, target): test merge to detect conflicts
- Returns list of conflicting files without modifying working directory
- runMergeCheck helper to capture stdout (where CONFLICT appears)
Tests cover clean merges, conflicting merges, and working dir cleanup.
Closes gt-3x1.2
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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