feat(mcp): expose ScriptedTool as MCP tool

[chaliy]

Summary

Integrate ScriptedTool with the existing MCP server so any ScriptedTool instance can be served over MCP's JSON-RPC protocol. Each ScriptedTool becomes one MCP tool (not one per sub-tool), preserving the code-mode benefit.

Motivation

The current MCP server (crates/bashkit-cli/src/mcp.rs) exposes a single generic bash tool. It has no awareness of ScriptedTool. Cloudflare's Code Mode MCP showed that collapsing N API endpoints into one code-accepting MCP tool dramatically reduces token cost and round-trips.

By exposing ScriptedTool over MCP, any MCP client (Claude Desktop, VS Code Copilot, Cursor, etc.) gets access to a sandboxed code-mode tool without custom integration work.

Design

MCP tools/list

Each registered ScriptedTool appears as one MCP tool:

{
  "tools": [
    {
      "name": "ecommerce_api",
      "description": "E-commerce API orchestrator. Write bash scripts using: get_user, list_orders, get_inventory, create_discount. Use `help <cmd>` for details.",
      "inputSchema": {
        "type": "object",
        "properties": {
          "commands": { "type": "string", "description": "Bash script using tool commands" }
        },
        "required": ["commands"]
      }
    }
  ]
}

MCP tools/call

Routes to ScriptedTool::execute():

{"method": "tools/call", "params": {"name": "ecommerce_api", "arguments": {"commands": "get_user --id 42 | jq '.name'"}}}

Implementation

1. Add McpServer::register_scripted_tool(tool: ScriptedTool) method
2. tools/list iterates registered ScriptedTools, using Tool::input_schema() and Tool::short_description()
3. tools/call dispatches by tool name, calls ScriptedTool::execute()
4. Keep existing bash tool for backward compat
5. Gate behind scripted_tool feature flag

Architecture

McpServer
├── bash tool (existing, always available)
├── ecommerce_api (ScriptedTool)
├── analytics_api (ScriptedTool)
└── ...

CLI

bashkit mcp --tool ecommerce.sh   # Load ScriptedTool from config/script
bashkit mcp                        # Just the generic bash tool (current behavior)

Acceptance criteria

• tools/list includes registered ScriptedTools
• tools/call routes to correct ScriptedTool and returns stdout/stderr/exit_code
• Error handling: unknown tool name → JSON-RPC error
• Existing bash tool unaffected
• Integration test: full JSON-RPC round-trip (initialize → tools/list → tools/call)
• Update spec 014 with MCP section

Effort

Medium — plumbing between existing MCP server and existing ScriptedTool. Main work is the routing and tool registration.