Expand description
Config file loading and layered merge.
Config::load() resolves three layers — global ~/.engram/engram.toml,
project-local <project>/.engram/engram.toml, and ENGRAM_* env
overrides — with priority env > project-local > global. This module
owns the file-tree side: it reads each TOML file into a generic
toml::Value tree, deep-merges the project-local tree over the global
one, and enforces the secret invariant that api_key values always come
from the global layer. config.rs keeps the Config shape and the
Config::load() orchestration that drives these functions.