feat: standardize exit codes, fix tests, apply pre-commit

[KHolodilin]

Description

Standardize CLI exit codes via a dedicated ExitCode enum, fix two failing tests, apply pre-commit fixes (line endings, ruff-format), and update documentation.

Closes #25

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update
• Other (please describe):

Related Issue

Closes #25

Changes Made

• Exit codes: Add email_processor.exit_codes.ExitCode enum (SUCCESS, PROCESSING_ERROR, VALIDATION_FAILED, FILE_NOT_FOUND, UNSUPPORTED_FORMAT, WARNINGS_AS_ERRORS, CONFIG_ERROR). CLI commands (config, imap, passwords, smtp, status) and __main__ return ExitCode values instead of raw ints.
• Tests: Fix test_full_cycle_send_folder (use isolated sent_files_dir, assert SMTP connect on ExitCode.SUCCESS) and test_create_default_config_example_not_found_with_rich_console (expect ExitCode.FILE_NOT_FOUND).
• Pre-commit: Normalize line endings (mixed-line-ending), ruff-format. All pre-commit hooks pass.
• Documentation: README key features and Exit Codes section (module ref, IntEnum, common scenarios 1 & 4); PLAN.md Issue chore: upgrade to Python 3.9, improve code quality tools and fix tests #5 marked implemented; exit_codes module docstring "See also" → README § Exit Codes.
• Scripts / .bat: Updated for subcommands (run, send folder/file), install_context_menu args, run_all_tests cov 95.

Testing

• All existing tests pass (718 passed, 32 skipped)
• New tests added for new functionality
• Manual testing performed (if applicable)
• Test coverage maintained or improved

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• Pre-commit hooks pass (pre-commit run --all-files)
• Code formatting passes (ruff format --check .)
• Linting passes (ruff check .)

Additional Notes

• create_default_config returns ExitCode.FILE_NOT_FOUND when the example config is missing; test_create_default_config_example_not_found_with_rich_console was updated accordingly.
• Full-cycle send_folder test uses an isolated sent_files_dir so it doesn't share sent-file state with other tests (same mock PDF hash could otherwise be marked sent).

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.26027% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
email_processor/cli/commands/smtp.py	92.30%	1 Missing ⚠️
email_processor/exit_codes.py	94.44%	1 Missing ⚠️

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