add tui for environment quick start

[wilsonwang371]

This pull request introduces two new design documents for the SPEAR quickstart tool, one in English and one in Chinese, and also includes several minor code improvements in the Rust codebase. The documents detail the architecture, goals, and best practices for the quickstart tool, while the code changes focus on style consistency and improved clarity.

New documentation:

• Added docs/quickstart-tool-design-en.md and docs/quickstart-tool-design-zh.md describing the extended quickstart tool's design, implementation status, goals, recommended practices, CLI shape, config spec, TUI UX, workflow, state and cleanup policy, security, and acceptance criteria. [1] [2]

Code improvements and refactoring:

Consistency and clarity in code style

• Improved formatting in conditional expressions and error messages for better readability in src/spearlet/backend_reporter.rs, src/spearlet/config.rs, src/spearlet/execution/ai/backends/openai_chat_completion.rs, and src/spearlet/execution/ai/backends/openai_realtime_ws.rs. [1] [2] [3] [4]
• Reformatted multi-line assertions and import statements for clarity in test modules. [1] [2]

Minor code fixes

• Fixed the order of backend constants in import statements for consistency in src/spearlet/backend_reporter.rs.
• Simplified logic for default model handling in src/spearlet/execution/ai/mod.rs.
• Improved registration message formatting in src/apps/keyword_filter_agent/main.rs.