feat: external tool / MCP plugin integration

[alpibrupa]

Problem

All 44 tools are local Lex CLI commands. There is no way to connect external APIs, CI systems, databases, package registries, or other MCP-compatible servers as tools available to agents.

Design

New module: src/tools/mcp.lex

type McpTool = { name: Str, description: Str, schema: JsonSchema, endpoint: Str }

fn load_mcp_tools(server_url: Str) -> [net] List[t.Tool]
fn call_mcp_tool(tool: McpTool, args: Json) -> [net] Json

Integration points:

• all_tools() in index.lex should optionally append MCP tools loaded from .lex/mcp.toml
• Permission rules need an mcp_tool(name) predicate in rules.lex so MCP tools go through the same gate
• The TUI should accept --mcp <url> to load tools from an MCP server at startup

Configuration (.lex/mcp.toml)

[[servers]]
url = "http://localhost:3000"
allow = ["search_issues", "create_pr"]

Acceptance

• MCP tools appear in all_tools() and are filterable via permission specs
• lex check --strict src/ passes
• Smoke test with a local MCP server stub