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/policy.ts

@@ -9,11 +9,13 @@ const PROFILE_TOOLS: Record<ToolProfile, Set<string>> = {
     'file.read',
     'file.list',
     'web.fetch',
+    'system.info',
   ]),
   messaging: new Set([
     'file.read',
     'file.list',
     'web.fetch',
+    'system.info',
     'memory.read',
     'memory.write',
     'memory.search',
@@ -23,6 +25,7 @@ const PROFILE_TOOLS: Record<ToolProfile, Set<string>> = {
     'file.read',
     'file.list',
     'web.fetch',
+    'system.info',
     'memory.read',
     'memory.write',
     'memory.search',