Multichain deployment with the keystore implementation for private keys

[anurag-p6]

it's a follow up pr from #21

Summary
Updated multichain deployment workflow to use Foundry keystore/account signing and added an example env for non-secret configuration.

Description

1. Refactored deployment flow (contracts/deployMultichain.sh) to support signer injection via Foundry flags (--account, --keystore, --password-file, --private-key for CI only).
2. Removed the dependency on loading PRIVATE_KEY from .env during on-chain deployments (solidity scripts now broadcast using the signer provided by forge CLI).
3. Added contracts/.env.example for required non-secret environment variables (RPC endpoints, verifier config, optional etherscan key).
4. Improved script readability for contributors (clearer separation of deployment vs ops usage and contributor-focused documentation).
5. Kept verifier deployment as an optional target (only required when deploying verifier).

Outcome & Benefit

1. Deploy safely on multiple networks without committing secrets to the repository.
2. Contributors can follow a consistent setup using RPC env vars + Foundry keystore/account.
3. Reduces deployment friction and common mistakes around private key handling (fewer wasted gas attempts and fewer failed runs).
4. Easier future upgrades to support additional networks/targets while keeping contributor workflow simple.

CA
Sepolia -
DeviceRegistry: 0xd08e0Dd610d8D44C695DDF5682194B2713D1b007
LensMintERC1155: 0x4BA1f83C905C58d3594b5d017C215699a4ddfe64
Base -
DeviceRegistry: 0xBD880bBed248C5A84042EBB5c234f12846f46436
LensMintERC1155: 0xDd730656B662981B63228C369c4dF569944c97f9
Arbitrum -
DeviceRegistry: 0xF5D0d541DdD7ee305065a05Ea91705e913602508
LensMintERC1155: 0xbED44b1766df95e7180C42f87aA03d89a2d8ebeB
Polygon -
insufficient balance 😢

SC of deployment

Closes #42