[BUG](cli): subcommand is unreachable due to incorrect list membership check

## Problem

In `chromadb/cli/cli.py` line 50:

```python
def app():
args = sys.argv
if ["chroma", "update"] in args:
update()
return
```

`["chroma", "update"] in args` checks if the **list** `["chroma", "update"]` is an element of `sys.argv`. Since `sys.argv` contains only strings, this is **always `False`** — the `update()` function can never be called.

When a user runs `chroma update`, `sys.argv` is `["chroma", "update"]`. The `in` operator checks if `["chroma", "update"]` (a list) is one of the elements — but the elements are `"chroma"` (str) and `"update"` (str), never a list. So the check silently falls through to the Rust CLI.

## Fix

Replace with a proper subcommand check:

```python
if args[1:2] == ["update"]:
```

Using slice `args[1:2]` avoids `IndexError` if `sys.argv` has only one element (no subcommand).

## Affected File

- `chromadb/cli/cli.py:50`

Start at app() in chromadb/cli/cli.py around line 50 and inspect how sys.argv dispatches subcommands. Verify that running chroma update calls update(), while invoking the CLI without a subcommand does not raise an IndexError and still follows the normal path.

Written by the indexing model from the issue text.