good first issue: document local quality commands in setup guide

[Ashutosh0x]

Add the contributor quality commands to docs/SETUP.md so new contributors do not need to hunt through CI files.

Acceptance criteria:

• Include cargo fmt, cargo test, cargo clippy, cargo deny, and cargo audit commands.
• Mention which checks are quick versus full-gate.
• Link to CONTRIBUTING.md.
• Keep commands copy-pasteable on Windows PowerShell where practical.

[zmo2s]

Hi! I’d like to take this issue if it’s available 🙂