chore: make local forge test workflow discoverable

[knoal]

Summary

• point Foundry at forge-test/ in foundry.toml so plain forge test discovers the suite
• add npm run test:forge as a documented local entrypoint
• document the local Foundry workflow in the README

Testing

• PATH=/home/alex/.foundry/bin:$PATH npm run test:forge

This is a small contributor-experience cleanup rather than a functional contract change.

[abhicris]

🤖 Audit verdict: safe

Documentation and configuration-only changes to enable Foundry test discovery; no code execution, new dependencies, credentials, or malicious payloads.

Audited by the kcolbchain PR pipeline. See pipeline docs.

[abhicris]

Merged — thank you, @knoal. Your first kcolbchain contribution is in. 🎉

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