[ai] Add instructions regarding constructor parameters (#15091)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.ai/instructions.md

@@ -124,6 +124,28 @@ This document provides essential context for AI models interacting with this pro
     *   **Indentation:** Use spaces (two per indentation level), not tabs
     *   **Type aliases:** Prefer `using type_t = int;` over `typedef int type_t;`
     *   **Line length:** Wrap lines at no more than 120 characters
+    *   **Constructor parameters vs setters:** Component properties that are both **required** and **invariant**
+        (never change after construction) should be constructor parameters rather than set via setter methods.
+        This makes the dependency explicit and prevents use of the object in an incompletely-initialized state.
+        In code generation, when calling `cg.new_Pvariable()` or the relevant helper function to create the component, pass these as arguments.
+        ```cpp
+        // Good - required invariant dependency as constructor parameter
+        class SourceTextSensor : public text_sensor::TextSensor, public Component {
+         public:
+          explicit SourceTextSensor(text::Text *source) : source_(source) {}
+         protected:
+          text::Text *source_;
+        };
+        ```
+        ```cpp
+        // Bad - required invariant dependency as setter
+        class SourceTextSensor : public text_sensor::TextSensor, public Component {
+         public:
+          void set_source(text::Text *source) { this->source_ = source; }
+         protected:
+          text::Text *source_{nullptr};
+        };
+        ```
 
 *   **Component Structure:**
     *   **Standard Files:**