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.

AGENTS.md

@@ -257,7 +257,16 @@ The dashboard embeds the real `hermes --tui` — **not** a rewrite.  See `hermes
 
 ## Adding New Tools
 
-Requires changes in **2 files**:
+For most custom or local-only tools, do **not** edit Hermes core. Use the plugin
+route instead: create `~/.hermes/plugins/<name>/plugin.yaml` and
+`~/.hermes/plugins/<name>/__init__.py`, then register tools with
+`ctx.register_tool(...)`. Plugin toolsets are discovered automatically and can be
+enabled or disabled without touching `tools/` or `toolsets.py`.
+
+Use the built-in route below only when the user is explicitly contributing a new
+core Hermes tool that should ship in the base system.
+
+Built-in/core tools require changes in **2 files**:
 
 **1. Create `tools/your_tool.py`:**
 ```python