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.

Non sono indicati file, test o punti di ingresso dell’implementazione. Inizia individuando la configurazione del worktree a livello di progetto della GitHub App e tracciando come produce l’attuale struttura repo_clone_path/copilot-worktrees/repo_clone_name/random-worktree-name. Prima dell’implementazione, chiarisci quali opzioni di personalizzazione e regole di denominazione sono necessarie; il lavoro completato dovrebbe includere una configurazione per progetto che produca in modo affidabile percorsi di worktree più brevi e prevedibili.

Scritto dal modello di indicizzazione a partire dal testo della issue.