nexo-plugin.toml manifest schema + validator.
Defines the TOML manifest contract every native Rust nexo plugin must ship.
Distinction vs crates/extensions/<n>/plugin.toml:
that schema describes subprocess tool extensions (stdio /
HTTP MCP servers spawned as separate processes). This schema
describes native Rust plugins that link into the daemon
and ship full mini-applications (agents + tools + skills +
channels + advisors + capability gates).