Modularize watcher runtime and reliability core - #2

What changed

• split the former single-file watcher into focused modules for configuration, errors, reliability controls, snapshot logic, browser integration, runtime orchestration, and utilities
• reduced biu_grade_watcher.py to a thin CLI and compatibility import surface
• preserved the existing command-line interface and public reliability primitives used by tests
• added a package module entry point and compatibility/import coverage
• updated the README to document the modular architecture and remove the monolith limitation

Why

The previous implementation kept browser integration, persistence, request pacing, circuit-breaker state, notifications, snapshot diffing, and orchestration in one large module. The split makes deterministic reliability logic independently understandable/testable and isolates the external BIU/Playwright boundary.

Validation target

GitHub Actions should compile the Python sources and run the deterministic reliability suite without contacting BIU or requiring credentials. Live browser/authentication behavior remains an environment-dependent integration boundary and is not represented as passing CI coverage.