feat: add prompt template system for LLM variable substitution

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/llm/__tests__/promptTemplate.test.ts

@@ -0,0 +1,61 @@
+import { describe, it, expect } from 'vitest';
+import { PromptTemplate, renderTemplate } from '../promptTemplate.js';
+
+describe('promptTemplate', () => {
+  it('renders a template with variable substitution', () => {
+    const template: PromptTemplate = {
+      name: 'test',
+      systemPrompt: 'You are a narrator for {{setting}}.',
+      userPrompt: '{{npcName}} is feeling {{mood}}. Describe their thoughts.',
+    };
+    const result = renderTemplate(template, {
+      setting: 'a medieval village',
+      npcName: 'Elara',
+      mood: 'anxious',
+    });
+    expect(result.system).toBe('You are a narrator for a medieval village.');
+    expect(result.user).toBe('Elara is feeling anxious. Describe their thoughts.');
+  });
+
+  it('leaves unmatched variables as-is', () => {
+    const template: PromptTemplate = {
+      name: 'test',
+      systemPrompt: 'Hello {{name}}, welcome to {{place}}.',
+      userPrompt: '',
+    };
+    const result = renderTemplate(template, { name: 'Brynn' });
+    expect(result.system).toBe('Hello Brynn, welcome to {{place}}.');
+  });
+
+  it('handles empty variables object', () => {
+    const template: PromptTemplate = {
+      name: 'test',
+      systemPrompt: 'No vars here.',
+      userPrompt: 'Also none.',
+    };
+    const result = renderTemplate(template, {});
+    expect(result.system).toBe('No vars here.');
+    expect(result.user).toBe('Also none.');
+  });
+
+  it('replaces multiple occurrences of same variable', () => {
+    const template: PromptTemplate = {
+      name: 'test',
+      systemPrompt: '{{name}} meets {{name}}.',
+      userPrompt: '',
+    };
+    const result = renderTemplate(template, { name: 'Thom' });
+    expect(result.system).toBe('Thom meets Thom.');
+  });
+
+  it('handles multiline templates', () => {
+    const template: PromptTemplate = {
+      name: 'test',
+      systemPrompt: 'Line 1: {{a}}\nLine 2: {{b}}',
+      userPrompt: '{{c}}',
+    };
+    const result = renderTemplate(template, { a: 'x', b: 'y', c: 'z' });
+    expect(result.system).toBe('Line 1: x\nLine 2: y');
+    expect(result.user).toBe('z');
+  });
+});

server/src/llm/promptTemplate.ts

@@ -0,0 +1,26 @@
+export interface PromptTemplate {
+  name: string;
+  systemPrompt: string;
+  userPrompt: string;
+}
+
+export interface RenderedPrompt {
+  system: string;
+  user: string;
+}
+
+export function renderTemplate(
+  template: PromptTemplate,
+  variables: Record<string, string>,
+): RenderedPrompt {
+  let system = template.systemPrompt;
+  let user = template.userPrompt;
+
+  for (const [key, value] of Object.entries(variables)) {
+    const pattern = `{{${key}}}`;
+    system = system.replaceAll(pattern, value);
+    user = user.replaceAll(pattern, value);
+  }
+
+  return { system, user };
+}