Add error-path + git-backed happy-path tests for the five previously untested ExternalMcpService tools: GetTaskWorktree, GetTaskDiff, MergeTask (dry-run + not-Done guard), ListWorktrees, CleanupTaskWorktree. Git-backed cases skip when git is unavailable. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>