plugin: run Cursor and VS Code's agent end to end #86

plugin/tests/e2e_harnesses.py runs Codex CLI, Copilot CLI and Gemini CLI against a stand-in model with no account. Cursor needs a signed-in account to run a model, and nothing here drives VS Code's agent. Both are covered only by fixtures from their documentation (plugin/tests/test_harnesses.py).

Still unverified there:

• how Cursor resolves a plugin hook's relative ./bin/hook.py, and that postToolUse's additional_context reaches the model;
• whether Cursor's third-party import also runs the Claude Code plugin's hooks;
• whether VS Code's terminal tool is really named run_in_terminal.

A run by someone with a Cursor seat and VS Code, with the plugin installed from plugin/cursor/README.md and plugin/copilot/README.md, settles all three.