Support configurable worktree paths per project

### Feature summary

Allow custom worktree location and naming per project

### What problem are you trying to solve?

Today, worktrees are created in a fixed structure like:

```text
repo_clone_path/
└── copilot-worktrees/
└── repo_clone_name/
└── random-worktree-name
```
For some projects, especially large C++ codebases, this can create very long filesystem paths. Those long paths can cause build or compilation issues or make the worktree harder to use in environments with strict path-length limits.

### Proposed solution

Allow users to customize the worktree location and naming structure on a per-project basis.

Possible improvements:

- Let users set a custom base worktree directory
- Let users customize the worktree folder structure
- Prefer shorter or more predictable generated worktree names
- Optionally support random-name generation patterns that keep paths compact

### Workflow impact

This would improve reliability for projects that are sensitive to path length, especially C++ and other build-heavy repositories. It would also make worktrees easier to manage for users who need a more compact or organization-specific directory layout.

### Installation context

Project-level worktree setup in the GitHub App

### Additional context

Current structure:
```text
repo_clone_path/
└── copilot-worktrees/
└── repo_clone_name/
└── random-worktree-name
```
A customizable structure would help avoid long path issues and make worktrees more accessible in constrained environments.

No files, tests, or implementation entry points are named. Start by locating the GitHub App's project-level worktree setup and tracing how it produces the current repo_clone_path/copilot-worktrees/repo_clone_name/random-worktree-name structure. Before implementation, clarify which customization options and naming rules are required; done should include per-project configuration that reliably produces shorter, predictable worktree paths.

Written by the indexing model from the issue text.