Function resolve_rtk_bin 

pub fn resolve_rtk_bin(override_value: Option<String>) -> String

Decide which rtk executable to invoke.

A blank value counts as unset. RTK_BIN= is how a shell commonly spells “clear this”, and env::var reports it as Ok("") rather than an error — so taking it literally means trying to spawn the empty string, which fails with a confusing OS error instead of quietly using rtk from PATH.

The variable is deliberately un-prefixed: the sibling rtk-mcp-agy and rtk-mcp-cc plugins honour the same one, and all three resolve it the same way, so relocating rtk takes one variable rather than three.