Add two tests:
- TestAddWithOptions_HasAgentsMD: verifies AGENTS.md exists in worktree
after creation when it's in git
- TestAddWithOptions_AgentsMDFallback: verifies fallback copy works when
AGENTS.md is not in git but exists in mayor/rig
Fixes: gt-sq1.3
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>