Function plan_prune 

pub fn plan_prune(
    paths: &DataPaths,
    now: SystemTime,
    max_idle: Duration,
    running_version: &str,
    current_pins: &[(String, String)],
    in_use: &InUse,
) -> Plan

Decide what could be removed. Reads only; nothing is touched.

current_pins: (provider, pin) for every provider in this build’s registry. running_version: this AgentMux’s version, whose legacy folder is kept.