docs: add YShell design documentation

[delichik]

Motivation

• Provide a consolidated design baseline for an open-source, cross-platform terminal client that follows Fluent Design and mimics Xshell workflows for implementation with Tauri + xterm.js.
• Capture product boundaries, UX decisions, IPC/architecture constraints and MVP scope to align contributors and avoid rework during engineering implementation.
• Define security, storage and testing expectations up front so platform-specific PTY/SSH responsibilities and sensitive-data handling are clear.

Description

• Add a design documentation set under docs/design containing README.md, product-requirements.md, ux-interaction.md, technical-architecture.md, and roadmap.md.
• product-requirements.md defines target users, core scenarios, MVP features, non-functional requirements and success metrics.
• ux-interaction.md specifies window/layout, session tree, tabs/splits, quick-connect flows, keyboard shortcuts and Fluent Design UI guidelines.
• technical-architecture.md documents the recommended directory layout, front-end/back-end responsibilities, IPC command/event patterns, data storage and SSH/PTY design, and roadmap.md provides phased milestones and risk mitigations.

Testing

• Ran git diff --check to validate no whitespace or simple diff issues, and the check passed.
• Ran find docs/design -type f -maxdepth 1 -print | sort | xargs -r wc -l to enumerate files and lines, confirming the new design files were added (total ~824 lines) and the check completed successfully.

---

Codex Task