make envstack the canonical entrypoint and deprecate envshell

[rsgalloway]

Currently, envshell is the primary way to drop into an interactive shell with a managed envstack environment. For usability and branding reasons, we should make envstack itself the canonical interactive entrypoint and deprecate envshell.

Proposed behavior

Running envstack with no arguments launches an interactive shell using the resolved environment:

$ envstack
🚀 Launching envstack shell... CTRL+D to exit
(prod) ~/dev/project$

Running envstack launches an interactive shell for that environment:

$ envstack dev
(dev) ~/dev/project$

• The shell launched should default to the user’s existing shell ($SHELL), falling back to /bin/bash if unavailable.
• set a default prompt string (see set default prompt string #82 )
• envshell should be deprecated:
• Remain as a thin wrapper around envstack
• Planned removal in a future major release