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.

tests/Unit/PromptLoaderTest.php

@@ -0,0 +1,36 @@
+<?php
+
+use App\Support\Ai\PromptLoader;
+
+test('load substitutes placeholders in the chat-system template', function () {
+    $prompt = PromptLoader::load('chat-system', [
+        'user_name' => 'Jane Doe',
+        'company_name' => 'Acme Corp',
+        'today' => '2026-04-11',
+    ]);
+
+    // Substituted values present
+    expect($prompt)->toContain('Jane Doe');
+    expect($prompt)->toContain('Acme Corp');
+    expect($prompt)->toContain('2026-04-11');
+
+    // Raw placeholders are gone
+    expect($prompt)->not->toContain('{{user_name}}');
+    expect($prompt)->not->toContain('{{company_name}}');
+    expect($prompt)->not->toContain('{{today}}');
+});
+
+test('load returns a trimmed template when no placeholders are provided', function () {
+    $prompt = PromptLoader::load('text-generation');
+
+    expect($prompt)->not->toBe('');
+    // No leading/trailing whitespace — trim() stripped the trailing newline.
+    expect($prompt)->toBe(trim($prompt));
+    // Sanity-check a word known to appear in the preamble.
+    expect($prompt)->toContain('writing assistant');
+});
+
+test('load throws when the template file is missing', function () {
+    expect(fn () => PromptLoader::load('definitely-not-a-real-prompt'))
+        ->toThrow(RuntimeException::class, 'Missing AI prompt template: definitely-not-a-real-prompt');
+});