Task manager + delegating the current thread to background

As an addendum to #3454, it would be fantastic if we had a "task manager" for M2. Here are the features I'd like to see first:
1. list all tasks, including created, running, finished, cancelled, and terminated tasks and some information like how long they have been running
2. connect stdio/stderr to a task to see its output
3. bring a task to foreground so that ctrl-c lands me in its debugger
4. retrieve the output of a finished task
5. cancel a runaway task

In the future, it would be great to also have the ability to enable/disable profiling on a thread after it has started to get some insight into what it is doing.

Additionally, one feature that I'm surprised I haven't seen before:
* if a running function in the current thread is taking too long, it would be great to delegate it to a background task, perhaps by a keyboard shortcut (ctrl-z?), so that it can be retrieved later.

Start by reading the related issue #3454 and the requested task-manager capabilities in this issue. No files, tests, or entry points are named, so first establish where task execution, stdio, debugger control, and keyboard handling live. Done requires an agreed scope and testable behavior for task listing, output access, foregrounding, cancellation, and background delegation.

Written by the indexing model from the issue text.