chore: add mainnet deploy script

[sainathr19]

Summary

No mainnet deploy script existed — HANDOFF.md's M4 steps were manual commands. Adds keeper/deploy-mainnet.mjs: declares + deploys EkuboAdapter then Iceberg using the same mainnet constants test_ekubo_fork.cairo already verified against real Ekubo liquidity.

Direction fixed to in_token=ETH, out_token=USDC (scheduled exit) — the exact direction the fork test exercises, so this carries no new, previously-untested behavior. INTERVAL_SECONDS defaults to 300 (5 min, short for initial testing) — worth knowing it's permanent once deployed, same as keeper/pool_address/swap_router (no setter exists on any of them).

Safety:

• Dry-run by default — prints exactly what it would deploy, touches nothing. Needs --yes to actually declare+deploy and spend gas.
• Reads in_token/out_token back from the deployed contract afterward and asserts they match intent, rather than trusting constructor calldata ordering blindly.
• DEPLOYER_PRIVATE_KEY only ever read from env / keeper/.env (already gitignored).
• mainnet-deployment.json (this script's output) gitignored to match the existing devnet-deployment.json convention — the durable public record belongs in strk20.json, not this handoff file.

Test plan

• node --check passes
• Dry-run path exercised end to end with dummy env values — prints the plan, exits 0, no network call made
• Not run for real — needs an actual funded deployer account, not available in this session. Whoever runs this for real should read the printed plan carefully before passing --yes.