Shared GitHub-install engine for tools and plugins. Holds the
generic install engine + [InstallManifest] trait, the install
error type, the install/identifier validators, and the install
whitelist. The per-kind discovery + wrappers live in
filesystem::tools and filesystem::plugins.
Local-filesystem tools — lightweight executables exposed to agents
(e.g. as MCP tools). A tool is either hand-placed under
<base_dir>/tools/ or fetched from GitHub via the shared install
engine (crate::filesystem::install); install holds the tool
side of that pipeline (impl-only, hence pub mod).