refactor: simplify template migration code

- Skip MiniJinja parse+render for plain-text strings (no {{ / {% / {#)
- Remove dead VariableExpansionTransform type alias
- Add From<TemplateError> for FabroError, replace manual map_err with ?
- Extract resolve_prompt_and_model helper in hooks executor

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/crates/fabro-hooks/src/executor.rs

@@ -96,6 +96,38 @@ impl HookExecutorImpl {
         }
     }
 
+    /// Resolve env vars in the prompt and optional model strings.
+    /// Returns `None` (with a warning) on resolution failure — callers should
+    /// proceed when that happens.
+    fn resolve_prompt_and_model<E>(
+        prompt: &str,
+        model: Option<&str>,
+        env: &E,
+        hook_kind: &str,
+    ) -> Option<(String, Option<String>)>
+    where
+        E: Env + ?Sized,
+    {
+        let prompt = match resolve_interp_string(prompt, env) {
+            Ok(prompt) => prompt,
+            Err(error) => {
+                tracing::warn!(error = %error, "{hook_kind} hook prompt env resolution failed, proceeding");
+                return None;
+            }
+        };
+        let model = match model
+            .map(|model| resolve_interp_string(model, env))
+            .transpose()
+        {
+            Ok(model) => model,
+            Err(error) => {
+                tracing::warn!(error = %error, "{hook_kind} hook model env resolution failed, proceeding");
+                return None;
+            }
+        };
+        Some((prompt, model))
+    }
+
     /// Execute a command hook (sandbox or host).
     async fn execute_command<E>(
         definition: &HookDefinition,
@@ -256,22 +288,9 @@ impl HookExecutorImpl {
     where
         E: Env + Clone + Send + Sync + fmt::Debug + 'static,
     {
-        let prompt = match resolve_interp_string(prompt, env) {
-            Ok(prompt) => prompt,
-            Err(error) => {
-                tracing::warn!(error = %error, "prompt hook prompt env resolution failed, proceeding");
-                return HookDecision::Proceed;
-            }
-        };
-        let model = match model
-            .map(|model| resolve_interp_string(model, env))
-            .transpose()
-        {
-            Ok(model) => model,
-            Err(error) => {
-                tracing::warn!(error = %error, "prompt hook model env resolution failed, proceeding");
-                return HookDecision::Proceed;
-            }
+        let Some((prompt, model)) = Self::resolve_prompt_and_model(prompt, model, env, "prompt")
+        else {
+            return HookDecision::Proceed;
         };
 
         let resolved_model = Self::resolve_model(model.as_deref());
@@ -323,22 +342,9 @@ impl HookExecutorImpl {
     where
         E: Env + Clone + Send + Sync + fmt::Debug + 'static,
     {
-        let prompt = match resolve_interp_string(prompt, env) {
-            Ok(prompt) => prompt,
-            Err(error) => {
-                tracing::warn!(error = %error, "agent hook prompt env resolution failed, proceeding");
-                return HookDecision::Proceed;
-            }
-        };
-        let model = match model
-            .map(|model| resolve_interp_string(model, env))
-            .transpose()
-        {
-            Ok(model) => model,
-            Err(error) => {
-                tracing::warn!(error = %error, "agent hook model env resolution failed, proceeding");
-                return HookDecision::Proceed;
-            }
+        let Some((prompt, model)) = Self::resolve_prompt_and_model(prompt, model, env, "agent")
+        else {
+            return HookDecision::Proceed;
         };
 
         let resolved_model = Self::resolve_model(model.as_deref());

lib/crates/fabro-template/src/lib.rs

@@ -124,7 +124,16 @@ impl From<minijinja::Error> for TemplateError {
     }
 }
 
+/// Returns `true` when the string contains no MiniJinja delimiters and can
+/// be returned as-is without paying for a full template parse+render cycle.
+fn is_plain_text(template: &str) -> bool {
+    !template.contains("{{") && !template.contains("{%") && !template.contains("{#")
+}
+
 pub fn render(template: &str, ctx: &TemplateContext) -> Result<String, TemplateError> {
+    if is_plain_text(template) {
+        return Ok(template.to_owned());
+    }
     let mut env = Environment::new();
     env.set_undefined_behavior(UndefinedBehavior::Strict);
     env.set_auto_escape_callback(|_| AutoEscape::None);

lib/crates/fabro-workflow/src/error.rs

@@ -337,6 +337,12 @@ impl From<GraphvizError> for FabroError {
     }
 }
 
+impl From<fabro_template::TemplateError> for FabroError {
+    fn from(err: fabro_template::TemplateError) -> Self {
+        Self::Validation(err.to_string())
+    }
+}
+
 impl From<fabro_validate::ValidationError> for FabroError {
     fn from(e: fabro_validate::ValidationError) -> Self {
         Self::Validation(e.0)

lib/crates/fabro-workflow/src/handler/agent.rs

@@ -80,7 +80,7 @@ pub(crate) fn expand_variables(
     let ctx = TemplateContext::new()
         .with_goal(graph.goal())
         .with_inputs(inputs.clone());
-    render_template(text, &ctx).map_err(|error| FabroError::Validation(error.to_string()))
+    Ok(render_template(text, &ctx)?)
 }
 
 /// Status fields that indicate a JSON object contains routing directives.

lib/crates/fabro-workflow/src/transforms/mod.rs

@@ -21,4 +21,3 @@ pub use model_resolution::ModelResolutionTransform;
 pub use preamble::PreambleTransform;
 pub use stylesheet_application::StylesheetApplicationTransform;
 pub use variable_expansion::TemplateTransform;
-pub type VariableExpansionTransform = TemplateTransform;

lib/crates/fabro-workflow/src/transforms/variable_expansion.rs

@@ -18,9 +18,7 @@ impl TemplateTransform {
     ) -> Result<(), FabroError> {
         for value in attrs.values_mut() {
             if let AttrValue::String(text) = value {
-                let rendered = render_template(text, ctx)
-                    .map_err(|error| FabroError::Validation(error.to_string()))?;
-                *text = rendered;
+                *text = render_template(text, ctx)?;
             }
         }
         Ok(())
@@ -30,8 +28,7 @@ impl TemplateTransform {
         let ctx = TemplateContext::new()
             .with_goal("{{ goal }}")
             .with_inputs(self.inputs.clone());
-        render_template(graph.goal(), &ctx)
-            .map_err(|error| FabroError::Validation(error.to_string()))
+        Ok(render_template(graph.goal(), &ctx)?)
     }
 }