studio: add --local to setup.sh + overlay unsloth-zoo from git main (#5252)

* studio: add --local to setup.sh + overlay unsloth-zoo from git main

setup.sh now accepts --local, which exports STUDIO_LOCAL_INSTALL=1 and
STUDIO_LOCAL_REPO=$REPO_ROOT. install_python_stack.py overlays unsloth-zoo
from git main on top of the editable unsloth checkout in both local_repo
branches (no-torch and with-torch).

The Colab notebook now invokes ./studio/setup.sh --local so the cloned
repo is used in editable mode and unsloth-zoo tracks main, matching the
behavior of install.sh --local on a VM. install.sh --local is unchanged:
it still sets SKIP_STUDIO_BASE=1, which short-circuits the local_repo
branches in install_python_stack.py, so the overlay is not run twice.

* studio: make --local overlays visible + guard empty arg parsing

- setup.sh: gate the --local flag loop on $# > 0 (defensive against any
  shell that surfaces unset $@ under set -u) and emit a substep when local
  mode is detected so the user can confirm the flag was parsed.
- install_python_stack.py: emit explicit _step lines before each overlay
  pip_install in both local_repo branches so overlays appear in the static
  log instead of being overwritten by the in-place progress bar.

studio/install_python_stack.py

@@ -972,6 +972,7 @@ def install_python_stack() -> int:
             req = REQ_ROOT / "no-torch-runtime.txt",
         )
         if local_repo:
+            _step(_LABEL, f"overlaying local repo (editable): {local_repo}")
             pip_install(
                 "Overlaying local repo (editable)",
                 "--no-cache-dir",
@@ -980,6 +981,15 @@ def install_python_stack() -> int:
                 local_repo,
                 constrain = False,
             )
+            _step(_LABEL, "overlaying unsloth-zoo from git main")
+            pip_install(
+                "Overlaying unsloth-zoo from git main",
+                "--no-cache-dir",
+                "--no-deps",
+                "--force-reinstall",
+                "unsloth-zoo @ git+https://github.com/unslothai/unsloth-zoo",
+                constrain = False,
+            )
     elif local_repo:
         # Local dev install: update deps from base.txt, then overlay the
         # local checkout as an editable install (--no-deps so torch is
@@ -994,6 +1004,7 @@ def install_python_stack() -> int:
             "unsloth-zoo",
             req = REQ_ROOT / "base.txt",
         )
+        _step(_LABEL, f"overlaying local repo (editable): {local_repo}")
         pip_install(
             "Overlaying local repo (editable)",
             "--no-cache-dir",
@@ -1002,6 +1013,15 @@ def install_python_stack() -> int:
             local_repo,
             constrain = False,
         )
+        _step(_LABEL, "overlaying unsloth-zoo from git main")
+        pip_install(
+            "Overlaying unsloth-zoo from git main",
+            "--no-cache-dir",
+            "--no-deps",
+            "--force-reinstall",
+            "unsloth-zoo @ git+https://github.com/unslothai/unsloth-zoo",
+            constrain = False,
+        )
     elif package_name != "unsloth":
         # Custom package name (e.g. roland-sloth for testing) — install directly
         _progress("base packages")