_load_config_impl now deep-merges the managed config.yaml on top of the
expanded user config so managed leaves win while sibling keys stay
user-controlled (leaf-level merge, D3). Managed values are expanded against
the process env only, never user-defined ${VAR}, so a user can't shadow a
managed literal. The managed file's (mtime,size) is folded into the load
cache key so editing it invalidates the cache. This inverts the usual
env-over-config precedence for pinned keys by design (see design doc §4.1).
