Harden Repo Verify command boundaries

[yashkhou]

Closes #8

What changed

• stop using a shell for internal Git commands and baseline refs
• add structured argv commands for acceptance gates
• add an argv-only policy that blocks legacy shell strings
• switch this repository self-check to argv-only commands
• add regression coverage for baseline shell injection and command policy

Verification

• npm test
• npm run benchmark
• npm run smoke
• Repo Verify self-check: 6/6 passed