Add CLAUDE.md with project documentation

- Add comprehensive project overview and architecture documentation
- Document CLI and TUI structure with reference to docs/cli_commands_structure.md
- Include testing patterns, fixtures, and CI configuration
- Document code style, conventions, and development guidelines
- Add known accounts, tracked accounts, and profile system details
- Reference configuration sources instead of hardcoded values

🤖 Generated with [Claude Code](https://claude.com/claude-code

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>