ci(deploy): scope testnet deploy to a GitHub Environment

[rubydusa]

Closes #41.

Summary

• Add environment input to _deploy-testnet.yml (default testnet).
• Add deploy-environment input to _foundry-cicd.yml (default testnet), forwarded to the orchestrator's deploy-testnet job.
• Deploy job declares environment: ${{ inputs.* }} so consumer repos can require reviewers and scope PRIVATE_KEY to the Environment.
• README documents how to set up the Environment in the consumer repo.

Why

Closes the gap between docs/specs/... (which calls for "deploy on approved PR to main") and the implementation. Today any PR with deploy-on-pr: true deploys without approval. With this change, the consumer repo's Environment protection rules (required reviewers, branch policy) are the gate.

Behavior change

• Default consumers must create a testnet Environment in their repo, or runs will sit waiting (or fail with "no environment found", depending on the GH config). This is intended — it's the point of the gate.
• Consumers who want the previous unscoped behavior can pass environment: '' / deploy-environment: ''.

Test plan

• All unit tests pass.
• YAML validates.
• CI green.
• Smoke test in a consumer repo with a testnet Environment configured.

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