[contracts] Add Foundry configuration for contract test commands

[FlowmemoryAI]

Objective

Add the smallest repo-level Foundry configuration needed so the existing RootfieldRegistry tests can run with a short, predictable command.

Allowed folders

• foundry.toml
• tests/README.md
• .github/workflows/ only if a lightweight non-deployment check is necessary

Forbidden folders

• apps/
• services/
• hardware/
• research/
• crypto/
• Production deployment scripts or network config

Acceptance criteria

• forge test or one documented command runs the existing contract tests from repo root.
• No RPC URLs, private keys, deployer config, production networks, or tokenomics are added.
• tests/README.md reflects the new command.
• Existing contract behavior remains unchanged.

Risk level

Low

Recommended worktree/agent

E:\FlowMemory\flowmemory-contracts / Protocol Contracts Agent

Scope guard

Keep this setup-only. Do not add product features, tokenomics, L1 work, or production deployment plumbing.