Changes the default CLI behavior when opening directories:

- `zed foo/` now adds `foo/` as a new workspace in the active window's
MultiWorkspace and opens the sidebar, instead of creating a new window.
If `bar/` was previously open, both `foo/` and `bar/` are retained as
separate workspaces in the sidebar.
- `zed -n foo/` continues to open a new window without the sidebar
(unchanged behavior).
- `zed -a foo/` continues to add to the existing workspace (unchanged
behavior).

The new behavior is gated on `multi_workspace_enabled` (the `agent-v2`
feature flag). Without the flag, directories open in a new window as
before, avoiding a situation where the workspace gets replaced with no
sidebar to switch back.

The change is contained to `open_paths()` in
`crates/workspace/src/workspace.rs`. When no CLI flag is specified and
no existing workspace matches the paths, we now look for the active
window and set `requesting_window` so that `Workspace::new_local` adds
the new workspace to it rather than creating a new window.

Release Notes:

- N/A