Feat(E0-01): Establish project skeleton and CI foundation

[DarkSamurai1907]

Closes #20.

This PR establishes the complete project skeleton for both physics_copilot and lean_proof_engine as defined in issue #20.

It also fully implements the CI/CD and tooling requirements from the "Tooling Setup" issue to ensure the new skeleton is ready for development.

Key Changes & Success Criteria Met

• Directory Structure: Created the full directory structures for physics_copilot and lean_proof_engine, including __init__.py files to ensure they import cleanly.
• Monorepo Setup: Implemented a root pyproject.toml using Hatch to manage all project dependencies, tools, and scripts.
• make dev Starts Services: Added a Makefile to physics_copilot and a dummy FastAPI app (backend/main.py) to fulfill the make dev requirement.
• CI Runs for Both Packages:
  • Added a .github/workflows/ci.yml file.
  • Configured ruff, mypy, and pytest in the root pyproject.toml.
  • Added test-cov script to enforce >80% coverage.
  • Scoped all linters and tools to only run on the physics_copilot and lean_proof_engine packages.
• Pre-Commit Hooks: Added a .pre-commit-config.yaml for local development quality.

All local checks (hatch run lint, hatch run typecheck, hatch run test-cov) are passing.