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>

tests/context/formatters/markdown-formatter.test.ts

@@ -103,7 +103,7 @@ describe('MarkdownFormatter', () => {
       const result = renderMarkdownHeader('my-project');
 
       expect(result).toHaveLength(2);
-      expect(result[0]).toBe('# [my-project] recent context');
+      expect(result[0]).toMatch(/^# \[my-project\] recent context, \d{4}-\d{2}-\d{2} \d{1,2}:\d{2}[ap]m [A-Z]{3,4}$/);
       expect(result[1]).toBe('');
     });
 
@@ -116,7 +116,7 @@ describe('MarkdownFormatter', () => {
     it('should handle empty project name', () => {
       const result = renderMarkdownHeader('');
 
-      expect(result[0]).toBe('# [] recent context');
+      expect(result[0]).toMatch(/^# \[\] recent context, \d{4}-\d{2}-\d{2} \d{1,2}:\d{2}[ap]m [A-Z]{3,4}$/);
     });
   });