[15] Dynamic plugin architecture

[rfunix]

Problem

Adding new tools requires modifying core Tengu code. There is no way for the community to distribute tools as independent pip packages.

Implementation Steps

1. Create `src/tengu/plugins/`:
   • `interface.py` — `TenguPlugin` Protocol with `name`, `version`, `tools`, `register(mcp)`
   • `loader.py` — Discovers plugins via `importlib.metadata.entry_points(group="tengu.plugins")` and local paths
   • `validator.py` — Validates plugin safety (no `shell=True`, required metadata present)
2. Add `[plugins]` section to `tengu.toml` config
3. Add `PluginsConfig` to `src/tengu/config.py`
4. Update `src/tengu/tools/registry.py` to also discover plugin tools
5. Create example plugin in `examples/tengu-plugin-example/` with proper `pyproject.toml` `[project.entry-points]`
6. Write tests

Plugin Interface

```python
class TenguPlugin(Protocol):
name: str
version: str
tools: list[callable] # List of @tengu_tool decorated functions

def register(self, mcp) -> None: ...

```

Files to Modify

• New: `src/tengu/plugins/init.py`, `interface.py`, `loader.py`, `validator.py`
• `src/tengu/tools/registry.py`
• `src/tengu/config.py`
• New: `examples/tengu-plugin-example/`

Dependencies

• [1] @tengu_tool decorator — Eliminate boilerplate #30 (`@tengu_tool` decorator)
• [2] Auto-registration of tools via discovery #32 (Auto-registration)

[rfunix]

Depends on: #30 (@tengu_tool decorator) and #32 (Auto-registration)