feat: add timestamp to context header

Display current date/time in context header for both terminal (Color)
and markdown formatters. Shows format like "2026-01-04 2:46am EST".

Also bumps version to 8.5.9.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugin/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem-plugin",
-  "version": "8.5.7",
+  "version": "8.5.9",
   "private": true,
   "description": "Runtime dependencies for claude-mem bundled hooks",
   "type": "module",