Function active 

pub fn active() -> Persona

Resolve the active persona from the loaded config (env > config > default).

This is the one entry point runtime consumers use (ctx_read mode resolution, ctx_url_read compression/trimming, sensitivity enforcement, MCP instructions). Resolution is cheap for built-ins (env read + match); only custom personas touch disk — same cost profile as Config::tool_profile_effective, which resolves the persona on every call today.