Add mainnet deployment checklist to docs/

[Ejirowebfi]

Description

The README mentions testing on testnet before mainnet but there is no formal checklist. Developers deploying to mainnet need a step-by-step guide to avoid costly mistakes.

Tasks

• Create docs/mainnet-deployment-checklist.md
• Include: contract audit, WASM optimization, fee configuration, admin key security, CSP header verification, smoke test on testnet, and rollback plan
• Add a link to the checklist in README.md under the Deployment section
• Add a GitHub issue template for "Mainnet Deployment" that references the checklist

Acceptance Criteria

• Checklist file exists with at least 10 actionable items
• Each item has a checkbox (- [ ])
• README.md links to the checklist
• GitHub issue template references the checklist URL