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>

crates/uv-cli/src/compat.rs

@@ -266,9 +266,6 @@ enum Resolver {
 /// These represent a subset of the `virtualenv` interface that uv supports by default.
 #[derive(Args)]
 pub struct VenvCompatArgs {
-    #[clap(long, hide = true)]
-    clear: bool,
-
     #[clap(long, hide = true)]
     no_seed: bool,
 
@@ -289,12 +286,6 @@ impl CompatArgs for VenvCompatArgs {
     /// behavior. If an argument is passed that does _not_ match uv's behavior, this method will
     /// return an error.
     fn validate(&self) -> Result<()> {
-        if self.clear {
-            warn_user!(
-                "virtualenv's `--clear` has no effect (uv always clears the virtual environment)"
-            );
-        }
-
         if self.no_seed {
             warn_user!(
                 "virtualenv's `--no-seed` has no effect (uv omits seed packages by default)"