Add a global default for MCP `codemode`

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Since MCP tools are often invoked with different arguments, routing all of them through Code Mode adds an extra layer to simple tool calls. It also leaves the agent with multiple ways to invoke a tool.

I would like a global default for codemode, with the per-server setting taking precedence. This would allow direct tools globally with selected servers opting into Code Mode, or Code Mode globally with selected servers opting out.

Currently, codemode has to be configured separately for every MCP server. A plugin can approximate this with ctx.mcp.transform, but this is a configuration preference and should not require a plugin.