Walks up the directory tree from a start directory looking for
.opencode/oh-my-openagent.json[c] (or legacy basename) files. Returns
detected paths in nearest-first order, optionally stopping at a
caller-provided directory.
Reuses detectPluginConfigFile so canonical/legacy basename detection,
caching, and JSONC vs JSON precedence stay consistent with existing
config loading. Foundation for #417 hierarchical config discovery.