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>

src/services/context/formatters/MarkdownFormatter.ts

@@ -14,12 +14,27 @@ import type {
 import { ModeManager } from '../../domain/ModeManager.js';
 import { formatObservationTokenDisplay } from '../TokenCalculator.js';
 
+/**
+ * Format current date/time for header display
+ */
+function formatHeaderDateTime(): string {
+  const now = new Date();
+  const date = now.toLocaleDateString('en-CA'); // YYYY-MM-DD format
+  const time = now.toLocaleTimeString('en-US', {
+    hour: 'numeric',
+    minute: '2-digit',
+    hour12: true
+  }).toLowerCase().replace(' ', '');
+  const tz = now.toLocaleTimeString('en-US', { timeZoneName: 'short' }).split(' ').pop();
+  return `${date} ${time} ${tz}`;
+}
+
 /**
  * Render markdown header
  */
 export function renderMarkdownHeader(project: string): string[] {
   return [
-    `# [${project}] recent context`,
+    `# [${project}] recent context, ${formatHeaderDateTime()}`,
     ''
   ];
 }
@@ -222,5 +237,5 @@ export function renderMarkdownFooter(totalDiscoveryTokens: number, totalReadToke
  * Render markdown empty state
  */
 export function renderMarkdownEmptyState(project: string): string {
-  return `# [${project}] recent context\n\nNo previous sessions found for this project yet.`;
+  return `# [${project}] recent context, ${formatHeaderDateTime()}\n\nNo previous sessions found for this project yet.`;
 }