feat(backend): MCP d'édition de templates (#81)

Ajoute le MCP dédié à l'édition de templates : catalogue et classification
des templates (mcp/templates.rs infrastructure + app-tauri), use cases et
provider (application/template), enforcement de la policy des tools (mod.rs,
server.rs, tools.rs), avec la parité côté chemin OpenAI-compatible
(openai_tools.rs x2).

Lots B1 (catalogue/classification), B2 (use cases/provider) et B3
(enforcement policy) livrés en un seul commit cohérent.

QA vert (seul l'échec de bind loopback #80, connu et non-régression, écarté).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/application/src/template/usecases.rs

@@ -134,9 +134,47 @@ impl UpdateTemplate {
 }
 
 // ---------------------------------------------------------------------------
-// ListTemplates / DeleteTemplate
+// ReadTemplate / ListTemplates / DeleteTemplate
 // ---------------------------------------------------------------------------
 
+/// Input for [`ReadTemplate::execute`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ReadTemplateInput {
+    /// Template to read.
+    pub template_id: TemplateId,
+}
+
+/// Output of [`ReadTemplate::execute`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ReadTemplateOutput {
+    /// The requested template.
+    pub template: AgentTemplate,
+}
+
+/// Reads one template from the global store.
+pub struct ReadTemplate {
+    templates: Arc<dyn TemplateStore>,
+}
+
+impl ReadTemplate {
+    /// Builds the use case.
+    #[must_use]
+    pub fn new(templates: Arc<dyn TemplateStore>) -> Self {
+        Self { templates }
+    }
+
+    /// Reads one template.
+    ///
+    /// # Errors
+    /// - [`AppError::NotFound`] if the template is unknown,
+    /// - [`AppError::Store`] on persistence failure.
+    pub async fn execute(&self, input: ReadTemplateInput) -> Result<ReadTemplateOutput, AppError> {
+        Ok(ReadTemplateOutput {
+            template: self.templates.get(input.template_id).await?,
+        })
+    }
+}
+
 /// Output of [`ListTemplates::execute`].
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ListTemplatesOutput {