Fix copilot instructions using the wrong commands

[jakebailey]

The instructions for Copilot were wrong/suboptimal.

• hereby --list doesn't exist. It's hereby --tasks. Fix that.
  • Once this works, we need good descriptions on the tasks. Add those, and hide tasks that nobody should be running at this point (and definitely not Copilot).
• hereby build doesn't build the project, it's just another way to build the tsgo binary, which is not needed for tests. Fix the description.
• Clarify the kinds of tests we use, to hopefully promote running Local for new tests.

[Copilot]

Pull Request Overview

This PR fixes incorrect and suboptimal instructions for Copilot by correcting command references and improving task descriptions. The changes ensure that Copilot provides accurate guidance to developers working with the TypeScript compiler port.

Key changes:

• Fixed the command to list available tasks from --list to --tasks
• Added descriptive text to build tasks to clarify their purpose and hide irrelevant tasks
• Enhanced test command documentation to better distinguish between submodule and local tests

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Herebyfile.mjs	Added descriptions to task definitions and hid internal tasks from the task list
.github/copilot-instructions.md	Updated command references and clarified build/test purposes