feat: queue user messages while busy

[Dhirenderchoudhary]

Description

Adds an in-memory queued user-message flow while the agent is busy. The chat input stays interactive during LLM streaming/tool execution,
submitted messages are queued FIFO, queued items can be selected with ↑/↓, edited with Enter, and removed with Ctrl+Delete.

Fixes #597

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Notes:

• npm run build passed
• npm run test:format passed
• npm run test:lint passed
• npm run test:knip passed
• Changed-area AVA passed: 69 tests
• node_modules/.bin/tsc --noEmit passed
• Full fail-fast AVA reached 406 passing tests, then stopped on existing timeout in source/auth/chatgpt-codex.spec.ts

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)