Scripts: Add a `deploy-all.sh` orchestrator for full deployment sequencing

[cybermax4200]

Labels: dx, scripts, good first issue

Problem:
The three contracts have a strict deployment dependency order: eco-token must be deployed first (its address is passed to reward-engine), then task-registry, then reward-engine. The current deploy.sh deploys only a single contract with no knowledge of this order. Contributors and CI have no turnkey way to do a full deployment.

Scope:

• Create scripts/deploy-all.sh that:
  1. Deploys eco-token, captures the contract ID
  2. Deploys task-registry, captures the contract ID
  3. Deploys reward-engine passing the two IDs
  4. Calls initialize on each contract in sequence
  5. Writes the resulting contract IDs to a .env.deployed file
• Add usage docs to the README's "Deploy to Testnet" section