Agents are basically separate "threads" with their own context window.

So the main claude can tell the test-runner agent "Run tests using `task test` and return the results"

Then the test-runner agent runs the tests, "wasters" its context by reading 500 lines of test results, sees that it's ok. Returns "tests ok" to the main context.

This way the main context is spared from the useless chatter and can go on for longer.