Feature request: Support a {repo} placeholder in gtr.worktrees.dir #205

Use case

I'd like to use a single global configuration to customize worktree base directories while keeping the repository name in each path.
For example, I'd like each repository's worktrees to live in a sibling directory named <repo>-workspace:

/repo/
├── foo/
├── foo-workspace/
│   └── feature-login/
├── bar/
└── bar-workspace/
    └── fix-bug/

Current behavior

In gtr v2.11.1, the default base directory is generated from the main repository's directory name as <repo>-worktrees.
An explicit gtr.worktrees.dir supports absolute paths, paths relative to the main repository root, and tilde expansion, but it does not support a placeholder for the repository name.

As a result, the layout above requires a separate configuration for each repository.
It would be useful to express this naming convention once in the global configuration.

Proposed behavior

Support a {repo} placeholder in gtr.worktrees.dir that expands to the main repository's directory name.

For example, this global Git configuration:

[gtr "worktrees"]
    dir = "../{repo}-workspace"

would resolve as follows:

I suggest defining {repo} as the basename of repo_root, using gtr's existing main repository discovery logic.
It would not be derived from the remote URL or the current linked worktree's directory name.
Running gtr from a linked worktree or a subdirectory would therefore resolve to the same main repository name.

Placeholder expansion would happen before the existing tilde expansion and relative-path resolution.
Relative paths would continue to be resolved from the main repository root.

Scope and compatibility

• Initially support only {repo}, without adding new configuration keys for a base-directory prefix or suffix.
• Keep the existing <repo>-worktrees default when no base directory is configured.
• Preserve existing paths that do not contain {repo}, as well as configuration precedence.
• Keep gtr.worktrees.prefix and --folder behavior unchanged; these would continue to control individual worktree directory names.
• Do not add automatic migration of existing worktrees.

Details such as handling existing paths that contain a literal {repo} and whether to provide an escape syntax could be discussed before implementation.

Questions

Would adding this placeholder to gtr.worktrees.dir fit the project's direction?
I'd also appreciate feedback on the {repo} syntax and the definition of the repository name above.

If this approach sounds reasonable, I'd be happy to prepare a PR with tests and documentation updates.