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.ts

@@ -0,0 +1,63 @@
+import os from 'os';
+import type { Tool, ToolResult } from '../types.js';
+
+function formatUptime(seconds: number): string {
+  const days = Math.floor(seconds / 86400);
+  const hours = Math.floor((seconds % 86400) / 3600);
+  const minutes = Math.floor((seconds % 3600) / 60);
+  return `${days}d ${hours}h ${minutes}m`;
+}
+
+function formatBytes(bytes: number): string {
+  return (bytes / (1024 * 1024 * 1024)).toFixed(2) + ' GB';
+}
+
+export const systemInfoTool: Tool = {
+  name: 'system.info',
+  description: 'Get current system information including date, time, hostname, OS, platform, architecture, uptime, Node.js version, and memory usage.',
+  inputSchema: {
+    type: 'object',
+    properties: {},
+    required: [],
+  },
+  execute: async (_rawArgs: unknown): Promise<ToolResult> => {
+    try {
+      const now = new Date();
+      const dateStr = now.toLocaleDateString('en-US', {
+        weekday: 'long',
+        year: 'numeric',
+        month: 'long',
+        day: 'numeric',
+      });
+      const timeStr = now.toLocaleTimeString('en-US', {
+        hour: '2-digit',
+        minute: '2-digit',
+        timeZoneName: 'short',
+        hour12: false,
+      });
+      const isoStr = now.toISOString();
+
+      const totalMem = os.totalmem();
+      const freeMem = os.freemem();
+
+      const lines = [
+        `Date:          ${dateStr}`,
+        `Time:          ${timeStr}`,
+        `ISO 8601:      ${isoStr}`,
+        `Hostname:      ${os.hostname()}`,
+        `Platform:      ${os.platform()}`,
+        `Architecture:  ${os.arch()}`,
+        `OS Release:    ${os.release()}`,
+        `Uptime:        ${formatUptime(os.uptime())}`,
+        `Node.js:       ${process.version}`,
+        `Memory Total:  ${formatBytes(totalMem)}`,
+        `Memory Free:   ${formatBytes(freeMem)}`,
+        `Working Dir:   ${process.cwd()}`,
+      ];
+
+      return { success: true, output: lines.join('\n') };
+    } catch (error) {
+      return { success: false, output: '', error: error instanceof Error ? error.message : String(error) };
+    }
+  },
+};