Native extensions and plugin migration

Rust applications should extend AGL by depending on the agl crate and registering handlers at startup. This is the stable native extension boundary:

let mut registry = agl::Registry::default();
registry.register("uppercase", |args, _agent| {
    let text = args["text"].as_str().unwrap_or_default();
    Ok(serde_json::json!({"text": text.to_uppercase()}))
});

Run the complete example with:

cargo run --example native_embed

AGL deliberately does not load Rust dynamic libraries. Rust does not provide a stable ABI for arbitrary trait objects and closures, so a libloading-style interface would couple plugins to an exact compiler and crate build. Compile-time registration is safer, easier to test, and works across supported platforms.

Existing Python plugins remain supported through --plugin. The Rust CLI discovers their task and tool registrations, then invokes handlers in isolated Python subprocesses over JSON. This bridge is intended for migration and requires python3 at runtime.

Migration steps:

1. Add agl and serde_json to the host Rust application.
2. Translate each Python handler into a Rust closure or function.
3. Register task handlers on Registry; register model-callable tools on ToolRegistry.
4. Run the existing .agent test blocks against the native registries.
5. Remove --plugin only after result and error behavior match.