Decouple remoteExecution runner plugin from py-slang's EV3Engine #151

Problem

src/runner/remoteExecution/index.ts (remoteRunnerPlugin) does two unrelated things:

1. Forwards data over a conductor channel (transport).
2. Constructs and runs py-slang's EV3Engine and handles the {type: 'run'} / {type: 'result'} protocol (execution).

Consequences:

• The package cannot be used without py-slang installed, so it is not the generic, reusable runner-side plugin it claims to be.
• It depends on a personal fork branch: github:proto-aiken-13/py-slang#branch-ev3-engine.
• It imports its message types (PySlangMessage, CHANNEL_ID, RUNNER_ID) from @sourceacademy/common-test, which looks like test scaffolding.

Proposal

Narrow the plugin to transport only, following the shape of CseMachinePlugin (pure channel forwarding, with the evaluator deciding when and what to send):

• Remove the EV3Engine import and all code-execution handling from this package.
• Move the run / result protocol to the evaluator side (py-slang), which owns EV3Engine.
• Expose a small send API on the plugin (e.g. sendConnectionStatus(status), mirroring CseMachinePlugin.sendSnapshots).
• Drop the py-slang dependency, and the worker-bundle rollup target if nothing else needs it.
• Add a changeset, since this renames/changes the public API.

Notes

• EV3 is currently working for the course, so this is a cleanup, not a blocker. Any change must keep that flow working, so the py-slang side should land first or together.
• Prior art: Branch remote runner plugin fixed #54 (closed, stale and conflicting) attempted this, along with Branch remote runner plugin #23 and Branch remote runner plugin fixe #46. It also added a new common-remote-execution package and a conductor resolutions pin; neither seems necessary for the core idea.