Add multi-environment mode and one-command setup feature

[Tharun-DV]

Add two major features to try:

1. Multi-environment mode (--base)

   • try init --base ~/src organizes workspaces into multiple
     environments (tries, experiments, prod) under a base directory
   • Two-step TUI: first pick an environment, then pick/create a project
   • Each subdirectory of the base path becomes an environment
   • New environments can be created on-the-fly by typing a name
   • ENV mode disables delete/rename (only projects get those)
   • Fully backward compatible: single-root mode unchanged

2. One-command setup (--install)

   • try init --base ~/src --install does everything:
     • Creates the base directory and default env subdirs
     • Auto-detects shell (zsh/bash/fish) from $SHELL
     • Appends shell function to the right config file
     • Skips if a try function is already present
   • All output goes to stderr (nothing on stdout) to prevent
     accidental eval side effects