[codex] Switch SDK defaults to production

[haochencheng]

Summary

• switch all SDK default environment resolvers from staging to prod
• update public README/docs/AI instructions/examples to use production domains by default
• keep staging documented only as a sandbox/E2E target with Arbitrum Sepolia + MockUSDC
• update repo hygiene checks so future docs stay prod-first

Production domains

• Website: https://www.synapse-network.ai
• SDK docs: https://docs.synapse-network.ai/sdks
• Gateway API: https://api.synapse-network.ai

Validation

• bash scripts/ci/pr_checks.sh
• bash scripts/ci/python_checks.sh
• bash scripts/ci/typescript_checks.sh
• bash scripts/ci/go_checks.sh
• bash scripts/ci/java_checks.sh
• bash scripts/ci/dotnet_checks.sh
• bash scripts/ci/repo_hygiene_checks.sh
• amem plan-check .
• amem docs-check . was run and still reports existing AMEM profile/metadata/contract gaps unrelated to this prod switch.

Security

• Diff-level scan found no private keys, JWTs, long agent keys, or private key payloads.