refactor(ai): move chat + text-gen prompts into resources/ai/prompts/

Extracts the two inline LLM prompts (AiAssistantService's chat system
prompt and AiTextGenerationService's writing preamble) out of PHP
heredocs and into plain-markdown template files under resources/ai/
prompts/. Each file can now be edited without opening a service
class, without wrestling with PHP string interpolation, and with
proper markdown syntax highlighting in editors.

A tiny PromptLoader helper at app/Support/Ai/PromptLoader.php reads
the file and does {{placeholder}} substitution via strtr() — no
Blade, because Blade's {{ $var }} HTML-escapes ampersands and quotes,
which is wrong for LLM prompts (a company called "Smith & Co" would
be sent as "Smith & Co"). Missing templates throw RuntimeException
so they fail loud during development.

Pure refactor: no prompt wording changes. Existing AI feature tests
(AiChatFlowTest, AiGenerationTest) pass unchanged — they assert on
message structure via ScriptedAiDriver, not on prompt content. Three
new unit tests in PromptLoaderTest lock the helper's contract:
placeholder substitution, no-var loading, missing-file error.

app/Support/Ai/PromptLoader.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace App\Support\Ai;
+
+use RuntimeException;
+
+/**
+ * Loads LLM prompt templates from disk and performs lightweight
+ * placeholder substitution.
+ *
+ * Prompts live as plain markdown files under `resources/ai/prompts/`
+ * so they can be edited without touching PHP service classes, get
+ * syntax-highlighted in editors, and produce clean PR diffs. Keeping
+ * the templates outside PHP also sidesteps heredoc indentation
+ * rules and interpolation quirks (dollar signs, curly braces).
+ *
+ * Placeholders use the `{{name}}` double-brace form — a common
+ * template convention that doesn't collide with markdown syntax and
+ * is trivial to substitute via strtr(). We deliberately do NOT use
+ * Blade for this: Blade's `{{ $var }}` HTML-escapes ampersands and
+ * quotes, which would corrupt prompts that reference names like
+ * "Smith & Co" (the model would see "Smith &amp; Co").
+ */
+class PromptLoader
+{
+    /**
+     * Load a prompt template and substitute any placeholders.
+     *
+     * @param  string  $name  Template filename without extension (e.g. 'chat-system').
+     * @param  array<string, scalar|\Stringable>  $vars  Placeholder => value map.
+     *
+     * @throws RuntimeException when the template file does not exist.
+     */
+    public static function load(string $name, array $vars = []): string
+    {
+        $path = resource_path("ai/prompts/{$name}.md");
+
+        if (! is_file($path)) {
+            throw new RuntimeException("Missing AI prompt template: {$name} (expected at {$path})");
+        }
+
+        $template = (string) file_get_contents($path);
+
+        if ($vars === []) {
+            return trim($template);
+        }
+
+        $replacements = [];
+        foreach ($vars as $key => $value) {
+            $replacements['{{'.$key.'}}'] = (string) $value;
+        }
+
+        return trim(strtr($template, $replacements));
+    }
+}