Add root-level validation workflow scripts

[Hobie1Kenobi]

Summary

Adds centralized validation workflow scripts to the root package.json for a more consistent contributor experience, as requested in #2646.

Changes

• lint: Runs lint across all packages via pnpm -r run lint
• typecheck: Runs type checking across all packages via pnpm -r run typecheck
• test: Replaces the placeholder echo "Error: no test specified" && exit 1 with pnpm -r run test to run tests across all packages
• validate: Unified validation workflow that runs lint, typecheck, and test in sequence

Motivation

Previously, contributors had to run package-level commands manually. These root-level scripts provide a consistent developer workflow and make it easy to validate the entire monorepo with a single command (pnpm run validate).

Closes #2646

Base payout address: 0x408f39B19266022FeC03076091e59D1f4f163658

Autonomous completion by Agentic Swarm Marketplace worker.

[vercel]

Someone is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1