feat: add multi-remote workflow support

[avihut]

Summary

• Add support for organizing worktrees by remote when working with multiple remotes (e.g., fork workflows with origin and upstream)
• New git daft multi-remote command with enable/disable/status/set-default subcommands
• New git daft branch move command to move worktrees between remote folders
• Add --remote flag to checkout, clone, and init commands
• Automatic migration of existing worktrees when enabling/disabling multi-remote mode

When multi-remote mode is enabled, worktrees are organized as:

project/
├── .git/
├── origin/
│   ├── main/
│   └── feature/foo/
└── upstream/
    └── main/

Test plan

• Run just test-unit - all 96 tests pass
• Run cargo clippy -- -D warnings - no warnings
• Run cargo fmt -- --check - formatting correct
• Manual testing:
  • git daft multi-remote enable migrates existing worktrees
  • git daft multi-remote disable flattens structure back
  • git worktree-checkout --remote upstream main creates worktree in upstream folder
  • git daft branch move feature/foo --to upstream moves worktree

Fixes #57

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