feat: add runtime context awareness — system.info tool + date/time in system prompt

- assembleSystemPrompt() now injects '# Runtime Context' with current date/time
- New system.info tool: date, time, hostname, platform, arch, uptime, memory, Node.js version
- Tool available in all profiles (minimal/messaging/coding/full)
- 983 tests passing (+7 new)

src/tools/builtin/system-info.test.ts

@@ -0,0 +1,84 @@
+import { describe, it, expect } from 'vitest';
+import { systemInfoTool } from './system-info.js';
+
+describe('system.info tool', () => {
+  // ── Metadata ─────────────────────────────────────────────────────────────
+
+  it('has correct metadata', () => {
+    expect(systemInfoTool.name).toBe('system.info');
+    expect(systemInfoTool.description).toBeTruthy();
+    expect(systemInfoTool.inputSchema.type).toBe('object');
+  });
+
+  // ── Execution ────────────────────────────────────────────────────────────
+
+  it('returns success with system info', async () => {
+    const result = await systemInfoTool.execute({});
+
+    expect(result.success).toBe(true);
+    expect(typeof result.output).toBe('string');
+    expect(result.output!.length).toBeGreaterThan(0);
+  });
+
+  it('output contains expected fields', async () => {
+    const result = await systemInfoTool.execute({});
+
+    expect(result.success).toBe(true);
+    const output = result.output!;
+
+    const expectedFields = [
+      'Date:',
+      'Time:',
+      'Hostname:',
+      'Platform:',
+      'Architecture:',
+      'Node.js:',
+      'Uptime:',
+      'Memory Total:',
+    ];
+
+    for (const field of expectedFields) {
+      expect(output).toContain(field);
+    }
+  });
+
+  it('output contains valid ISO timestamp', async () => {
+    const result = await systemInfoTool.execute({});
+
+    expect(result.success).toBe(true);
+    const output = result.output!;
+
+    // Find the line containing 'ISO 8601:'
+    const isoLine = output.split('\n').find((line) => line.includes('ISO 8601:'));
+    expect(isoLine).toBeTruthy();
+
+    // Extract the ISO string and validate it
+    const isoMatch = isoLine!.match(/ISO 8601:\s*(.+)/);
+    expect(isoMatch).toBeTruthy();
+
+    const isoString = isoMatch![1].trim();
+    const parsed = new Date(isoString);
+    expect(parsed.toISOString()).toBe(isoString);
+  });
+
+  it('output contains working directory', async () => {
+    const result = await systemInfoTool.execute({});
+
+    expect(result.success).toBe(true);
+    expect(result.output).toContain('Working Dir:');
+  });
+
+  // ── Idempotency ──────────────────────────────────────────────────────────
+
+  it('handles repeated calls', async () => {
+    const result1 = await systemInfoTool.execute({});
+    const result2 = await systemInfoTool.execute({});
+
+    expect(result1.success).toBe(true);
+    expect(result2.success).toBe(true);
+    expect(typeof result1.output).toBe('string');
+    expect(typeof result2.output).toBe('string');
+    expect(result1.output!.length).toBeGreaterThan(0);
+    expect(result2.output!.length).toBeGreaterThan(0);
+  });
+});