feat: auto-install Python deps from template pyproject.toml via uv

ExternalToolManager.discover() now accepts template root dir, detects
pyproject.toml and runs `uv sync` to create a venv. Tool invocation and
schema discovery inject the venv PATH/VIRTUAL_ENV so template tools can
import declared dependencies without manual installation.

src/template.rs

@@ -550,8 +550,7 @@ impl LoadedTemplate {
             .await
             .unwrap_or_default();
 
-        let tools_dir = base.join("tools");
-        let external_tools = ExternalToolManager::discover(&tools_dir).await;
+        let external_tools = ExternalToolManager::discover(base).await;
         tracing::info!("Template '{}': {} external tools", template_id, external_tools.len());
 
         let kb_dir = base.join("kb");