feat: graduate wait-what into the productivity bucket

Moves the skill out of in-progress/ and ships it: bucket README,
top-level README, plugin.json, an ask-matt router entry, and a docs
page at docs/productivity/wait-what.md.

Productivity rather than engineering — it fires in any conversation
with an agent, code or not. The CONTEXT.md clause is an opportunistic
hook, not a prerequisite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -42,6 +42,7 @@
     "./skills/productivity/handoff",
     "./skills/productivity/teach",
     "./skills/productivity/to-questionnaire",
+    "./skills/productivity/wait-what",
     "./skills/productivity/writing-for-agents"
   ]
 }