Add agent mode: autonomous file editing with automatic test running

[livioso]

This commit introduces a new /agent mode that works like Claude Code, providing autonomous file editing and test running capabilities.

Features:

• Automatic file identification and editing without manual approval
• Automatic test execution after each edit cycle
• Iterative fixing of test failures
• Configurable maximum iteration limit (default: 3)
• Uses SEARCH/REPLACE block format for edits

Implementation:

• Created AgentCoder class extending EditBlockCoder
• Created AgentPrompts with agent-specific system prompts
• Added /agent slash command for entering agent mode
• Added agent mode to chat mode list
• Comprehensive test suite in test_agent.py

Usage:

• /agent - Execute a request in agent mode
• /chat-mode agent - Switch to agent mode
• Configure with --max-agent-iterations and --auto-test flags

The agent mode enables workflows where the AI can autonomously:

1. Analyze the request
2. Make necessary file edits
3. Run tests automatically
4. Analyze failures and fix issues
5. Iterate until tests pass or max iterations reached

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