Update Install Scripts (#5968)

* Update Install Scripts

Add SPDX AGPL-3.0 headers to the installer scripts and let the piped web
installs take their common options from the environment.

- install.sh / install.ps1: read UNSLOTH_NO_TORCH (and UNSLOTH_PYTHON for
  install.sh) so a piped install needs no positional flags. Flags and the
  pipe forms still work; an explicit flag wins.
- Fix the UNSLOTH_STUDIO_HOME example so the variable sits after the pipe
  and reaches sh instead of curl.
- Add SPDX headers to install.sh, install.ps1, the uninstall scripts, and
  the MLX install scripts.
- Drop the internal test package names from the studio install comments.

* Mirror UNSLOTH_PYTHON env var to install.ps1

install.ps1 now reads UNSLOTH_PYTHON to pin the Python version, matching
install.sh, and lists all three env vars (UNSLOTH_NO_TORCH, UNSLOTH_PYTHON,
UNSLOTH_STUDIO_HOME) in the header examples. The requested version is
preferred during detection and used as the winget install target; behavior
is unchanged when the variable is unset.

studio/install_python_stack.py

@@ -1406,7 +1406,7 @@ def install_python_stack() -> int:
     # When called from "unsloth studio update", it is NOT set so base packages
     # (unsloth + unsloth-zoo) are always reinstalled to pick up new versions.
     skip_base = os.environ.get("SKIP_STUDIO_BASE", "0") == "1"
-    # When --package is used, install a different package name (e.g. roland-sloth for testing)
+    # When --package is used, install a different package name (for testing)
     package_name = os.environ.get("STUDIO_PACKAGE_NAME", "unsloth")
     # When --local is used, overlay a local repo checkout after updating deps
     local_repo = os.environ.get("STUDIO_LOCAL_REPO", "")
@@ -1565,7 +1565,7 @@ def install_python_stack() -> int:
             constrain = False,
         )
     elif package_name != "unsloth":
-        # Custom package name (e.g. roland-sloth for testing) — install directly
+        # Custom package name (for testing), install directly
         _progress("base packages")
         pip_install(
             f"Installing {package_name}",