Update the README quick-overview bullet and the dedicated File Locations
section in docs/reference/configuration.md to describe the walk-up
behaviour added in #417: configs under `.opencode/` are discovered by
walking from the working directory up to $HOME, with closer configs
winning.
Includes a hierarchical example (`~/.config/opencode/` global,
`~/work/.opencode/` work overrides, repo-specific overrides under that)
and a security note explaining why `mcp_env_allowlist` remains
extensible only from the canonical user config.
Matan Kushner
13c70cff12