chore(pre-commit): add pre-push pytest coverage hook

[JTJag]

Summary

Adds a CI-parity pre-push test gate in pre-commit and fixes .env.example to avoid invalid session parsing during test collection.

Changes

• Added local pre-push hook:
  • pytest-coverage-ci
  • command: uv run --env-file .env.example pytest --cov --cov-report=term-missing --cov-report=xml
• Updated .env.example:
  • switched TELEGRAM_SESSION_STRING from active invalid placeholder to commented example
  • keeps TELEGRAM_SESSION_NAME as the default test-friendly session mode

Why

• Tests were recently added and should be enforced before push as well.
• Coverage threshold is already configured in pyproject.toml (fail_under = 80), so this hook enforces the same quality gate locally.
• Loading .env.example previously could trigger StringSession parsing errors due to placeholder session string content.

Validation

• pre-commit run pytest-coverage-ci --hook-stage pre-push --all-files passes.
• Hook now mirrors CI command style via uv run.