Require uv venv --clear before removing an existing directory (#14309)

By default, `uv venv <venv-name>` currently removes the `<venv-name`>
directory if it exists. This can be surprising behavior: not everyone
expects an existing environment to be overwritten. This PR updates the
default to fail if a non-empty `<venv-name>` directory already exists
and neither `--allow-existing` nor the new `-c/--clear` option is
provided (if a TTY is detected, it prompts first). If it's not a TTY,
then uv will only warn and not fail for now — we'll make this an error
in the future. I've also added a corresponding `UV_VENV_CLEAR` env var.

I've chosen to use `--clear` instead of `--force` for this option
because it is used by the `venv` module and `virtualenv` and will be
familiar to users. I also think its meaning is clearer in this context
than `--force` (which could plausibly mean force overwrite just the
virtual environment files, which is what our current `--allow-existing`
option does).

Closes #1472.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

docs/reference/environment.md

@@ -458,6 +458,11 @@ Equivalent to the `--torch-backend` command-line argument (e.g., `cpu`, `cu126`,
 Used ephemeral environments like CI to install uv to a specific path while preventing
 the installer from modifying shell profiles or environment variables.
 
+### `UV_VENV_CLEAR`
+
+Equivalent to the `--clear` command-line argument. If set, uv will remove any
+existing files or directories at the target path.
+
 ### `UV_VENV_SEED`
 
 Install seed packages (one or more of: `pip`, `setuptools`, and `wheel`) into the virtual environment