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/index.ts

@@ -14,4 +14,5 @@ export { fileReadTool } from './builtin/file-read.js';
 export { fileWriteTool } from './builtin/file-write.js';
 export { fileEditTool } from './builtin/file-edit.js';
 export { fileListTool } from './builtin/file-list.js';
+export { systemInfoTool } from './builtin/system-info.js';
 export { webFetchTool } from './builtin/web-fetch.js';