feat(context): add dsh executor and headroom active loopback probe

Wire DeepSeek Harness as an optional fourth code executor (default off)
and add a loopback observation script that compares prompt tokens through
headroom proxy with OPENAI_TARGET_API_URL pointed at the compat proxy.

Co-authored-by: Cursor <cursoragent@cursor.com>

executor-display-label.mjs

@@ -2,6 +2,7 @@ const EXECUTOR_DISPLAY_LABELS = Object.freeze({
   cursor: 'TKMind 智趣',
   aider: 'Aider',
   openhands: 'OpenHands',
+  dsh: 'DeepSeek Harness',
   goose: 'TKMind',
 });