ci: add repository validation workflow

[JustinPerea]

What changed

• Adds three independent GitHub Actions jobs for backend, frontend, and contract validation.
• Runs on pull requests and pushes to main.
• Pins official checkout/setup actions to immutable commits.
• Keeps workflow permissions read-only and does not change branch protection.

Gates

• Backend: Python 3.12 dependency install plus full pytest suite.
• Frontend: Node.js 22 clean npm ci, tests, lint, and production build.
• Contracts: node contract check, provider contract inventory, and generated model reference check.

Local validation

• Merged-main baseline: 1,146 backend tests passed with two existing AsyncMock warnings.
• 392 frontend tests, lint, and production build passed.
• 142 node contracts, 71-node inventory, and generated reference passed.
• Workflow YAML parsed successfully with exactly the three expected jobs.
• git diff --check and staged credential scans passed.
• No paid provider calls ran.