Fix default branch/remote pathway

[bjester]

Summary

Running gh-worktree create some-branch should have been basing the branch off the default remote and default branch of the repo on Github. It was erroring out and this fixes that.

Traceback (most recent call last):
  File "__main__.py", line 4, in <module>
  File "gh_worktree/cli.py", line 7, in main
  File "fire/core.py", line 135, in Fire
  File "fire/core.py", line 468, in _Fire
  File "fire/core.py", line 684, in _CallAndUpdateTrace
  File "gh_worktree/commands/create.py", line 40, in __call__
  File "gh_worktree/runtime.py", line 33, in get_remote
ValueError: Must provide either owner_name or name
[PYI-164413:ERROR] Failed to execute script '__main__' due to unhandled exception!

[github-actions]

🎉 Build Assets are ready!

Type	File	Checksum
Wheel	gh_worktree-0.3.0-py3-none-any.whl	a755256dbe2fef2d7b373f2e8e3cf927d834b595e365c639d371828c3f89ba50
PEX	gh-worktree.pex	9a01b051ed29ca1308be2257631a0aa89575ce224a5f0067bd0260e61ae6923e
Binary	gh-worktree	c085903bb91cc6964fd8a5fc7c9e5cb76b8c19bcea9b8cd5d00a4b569955f6ad

[gemini-code-assist]

Code Review

This pull request updates the create command to use a default remote when one is not explicitly provided. The test suite has been updated to reflect this change, including the addition of a test case for branch-only references. A review comment suggests improving test robustness by explicitly asserting that the default remote logic is bypassed when a remote is provided in the reference.