Support Bun runtime as alternative to Node.js 18+

Summary

Allow Claude Code to run with Bun as an alternative to the Node.js 18+ requirement.

Context

• Anthropic acquired Bun in December 2025
• Claude Code already ships as a Bun executable internally
• Users who have Bun installed but not Node.js 18+ currently cannot run Claude Code (e.g., in VS Code extension output)

Proposal

Detect Bun runtime as a valid alternative when checking runtime requirements, instead of strictly requiring Node.js 18+.

Benefits

• Aligns with Anthropic's investment in Bun
• Provides flexibility for developers who prefer Bun
• Bun offers faster startup times and better performance

<img width="1561" height="899" alt="Image" src="https://github.com/user-attachments/assets/b827d8e5-fdfc-42b7-90e6-520c097b0375" />