feat: Implement aishell - AI-powered shell automation tool

Implemented a generic alternative to Claude Code with the following features:

Core Implementation:
- Multi-LLM provider support (OpenAI compatible APIs)
- Function calling for direct tool execution by LLM
- Interactive REPL shell interface
- MCP server mode for Claude Desktop integration
- Shell executor with bash, read, write, list tools

Architecture:
- src/cli: Interactive REPL implementation
- src/llm: LLM provider abstraction (OpenAI compatible)
- src/shell: Shell execution engine with duct
- src/mcp: MCP server for Claude Desktop
- src/config: Configuration management

Technical Stack:
- Rust 2021 with tokio async runtime
- clap for CLI framework
- reqwest for HTTP client
- duct for shell execution
- rustyline for REPL interface

This tool integrates with aigpt to form AIOS (AI Operating System),
enabling AI-driven OS management and automation.

Based on aigpt architecture for CLI and MCP patterns.

.gitignore

@@ -0,0 +1,21 @@
+# Rust
+/target/
+Cargo.lock
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Logs
+*.log
+
+# Environment
+.env
+.env.local