Reintegrate with `conda activate`

This package has been serving as experiment to run Pixi-like activation in new shells. I think we can consider it's useful and I'd like to see this integrated in `conda` natively. It's unlikely we can deprecate `conda activate` and teach users to use `conda spawn` or `conda shell`. Muscle memory is strong, after all.

My plan would be to:

- In unitialized interactive shells, `conda activate` runs like `python -m conda spawn`. This is, `activate` would create a new shell process with the right ephemeral initialization for that session. `deactivate` would prompt the user to `exit` or Ctrl+D. For clarity, this same logic can be made available as an additional subcommand, `shell`, but this may add more confusion, so maybe leave it out.
- In non-interactive shells (e.g. inside a script), we error out asking users to run the shell hook via `python -m conda shell.bash hook` or similar.
- In initialized installations, where `conda` is a shell function that forwards to shell subcommands or Python subcommands as needed, things would keep working as always. The shell `activate` subcommand shadow the `spawn`-like one implemented in Python.

Start by reading the existing behavior around `conda activate`, `python -m conda spawn`, and the `python -m conda shell.bash hook` entry point. Define how initialized and uninitialized interactive shells should differ, how non-interactive shells should fail, and how `deactivate` should behave; done requires an agreed design and corresponding implementation across supported shells.

Written by the indexing model from the issue text.