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.

src/main.rs

@@ -0,0 +1,74 @@
+use anyhow::Result;
+use clap::{Parser, Subcommand};
+use tracing_subscriber;
+
+use aishell::cli::Repl;
+use aishell::mcp::MCPServer;
+
+#[derive(Parser)]
+#[command(name = "aishell")]
+#[command(about = "AI-powered shell automation - A generic alternative to Claude Code")]
+#[command(version)]
+struct Cli {
+    #[command(subcommand)]
+    command: Commands,
+}
+
+#[derive(Subcommand)]
+enum Commands {
+    /// Start interactive AI shell
+    Shell {
+        /// LLM provider (openai, anthropic, ollama)
+        #[arg(short, long, default_value = "openai")]
+        provider: String,
+
+        /// Model name
+        #[arg(short, long)]
+        model: Option<String>,
+    },
+
+    /// Execute a single command via AI
+    Exec {
+        /// Command prompt
+        prompt: String,
+
+        /// LLM provider
+        #[arg(short = 'p', long, default_value = "openai")]
+        provider: String,
+    },
+
+    /// Start MCP server (for Claude Desktop integration)
+    Server,
+}
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    // Initialize logging
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::from_default_env()
+                .add_directive(tracing::Level::INFO.into()),
+        )
+        .init();
+
+    let cli = Cli::parse();
+
+    match cli.command {
+        Commands::Shell { provider, model } => {
+            let mut repl = Repl::new(&provider, model.as_deref()).await?;
+            repl.run().await?;
+        }
+
+        Commands::Exec { prompt, provider } => {
+            let mut repl = Repl::new(&provider, None).await?;
+            repl.execute_once(&prompt).await?;
+        }
+
+        Commands::Server => {
+            let server = MCPServer::new()?;
+            server.run().await?;
+        }
+    }
+
+    Ok(())
+}