docs: default custom tool creation to plugins

Steers custom tool creation toward the plugin route by default.
The adding-tools.md guide is now explicitly for built-in core Hermes
tools only.

Key fixes:
- Plugin quickstart: ctx.register_tool() now uses correct keyword-arg
  API (name=, toolset=, schema=, handler=) instead of broken 3-arg call
- Handler signature: (params, **kwargs) instead of (params)
- Handler return: json.dumps({...}) instead of plain string
- AGENTS.md: mentions plugin route before built-in tool instructions
- learning-path.md: plugins listed before core tool development
- contributing.md: separates plugin vs core tool paths

Based on PR #13138 by @helix4u.

website/docs/developer-guide/contributing.md

@@ -22,7 +22,8 @@ We value contributions in this order:
 
 ## Common contribution paths
 
-- Building a new tool? Start with [Adding Tools](./adding-tools.md)
+- Building a custom/local tool without modifying Hermes core? Start with [Build a Hermes Plugin](../guides/build-a-hermes-plugin.md)
+- Building a new built-in core tool for Hermes itself? Start with [Adding Tools](./adding-tools.md)
 - Building a new skill? Start with [Creating Skills](./creating-skills.md)
 - Building a new inference provider? Start with [Adding Providers](./adding-providers.md)