Reuse existing Zed window when opening worktrees

[npkanaka]

Summary

• Pass Zed's --existing flag when opening local Zed and Zed Preview worktrees.
• Reuse an exact already-open project window instead of creating a duplicate.
• Update the launcher regression test.

Closes #805

Validation

• Parsed SupacodeSettingsShared/Domain/OpenWorktreeAction.swift with swiftc.
• Parsed supacodeTests/OpenWorktreeActionTests.swift with swiftc.
• Full Xcode test/build is pending: this machine has only Command Line Tools active, not Xcode.

[github-actions]

Thanks for the pull request. It doesn't meet the contribution policy yet, so I've labeled it invalid. Please address the following, then push an update:

• Linked issue Opening an already-open Zed worktree creates a duplicate window #805 is not marked ready yet. A maintainer adds ready once the issue is triaged and approved for a pull request.

Nothing needs to be redone. A pull request left invalid is closed automatically after a few days of inactivity. See CONTRIBUTING.md for the full flow.