feat: add register-tool reference plugin

[Osraka]

Summary

• add plugin-tool-example, a minimal reference plugin for ctx.register_tool()
• demonstrate a single deterministic text_stats tool with JSON Schema, input validation, and structured tool_result() / tool_error() responses
• list the new example in the repo README and pair it with the existing plugin-authoring guide

Why

The repo already has reference examples for ctx.llm and dashboard plugins, and there is an open hook example PR, but it does not yet include the smallest copyable example for the core model-callable tool surface. This keeps the showcase balanced without turning the example into a full plugin tutorial.

Test plan

• PYTHONPATH=../hermes-agent python3 -m py_compile plugin-tool-example/__init__.py
• smoke-called _handle_text_stats() locally with a short sample payload