docs: add upstream verification test checklist

script/upstream/VERIFICATION_TEST.md

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+# Verification Test
+
+Use this checklist to verify CLI and VS Code extension behavior after upstream merge work.
+
+## CLI
+
+- Ask which model it is using.
+- Ask it to use two subagents:
+  - One subagent runs `git status`.
+  - The other subagent runs either `ls` or `npm install`.
+  - Change settings if needed to trigger a permission prompt.
+- Quickly ask it a simple follow-up question, such as `What is 2 + 2?`, to verify queued messages work.
+- Ask `What is my favourite animal?` and have it provide options.
+- Change the model and ask it about the favourite animal again.
+- Find `/local-review` and run it.
+- Change from Code mode to Ask mode and ask it what it can do.
+
+## VS Code Extension
+
+Run all CLI verification steps in the VS Code extension, then verify the extension-specific flows:
+
+- Use the diff button in the sidebar next to the Worktree button.
+- Use the Worktree button.
+- Open history, select a previous conversation, and ask `What's the last thing I asked you?`.
+- Create a worktree and ask `In which branch are you?`.
+- Open the diff viewer, make a comment, and send it to chat.
+- Run permissions on a subagent in a worktree.
+- Ask it to read and edit a file by adding a random line.