Add configuration to include org/owner name in project's default display name

### Feature summary

Add a setting to optionally use the full `owner/repo` name as a project's default display name in the Copilot app.

### What problem are you trying to solve?

Projects currently display only the repository name (e.g. `app`), dropping the owner/org. When a user works across multiple organizations that have same-named repositories (e.g. `github/app` vs `acme/app`), the sidebar shows ambiguous, duplicate-looking project entries. There is no built-in way to disambiguate them short of manually renaming each project.

### Proposed solution

Provide a configuration option (global preference, and/or per-project) that controls how a project's default display name is derived:

- Default (current): repo name only — `app`
- New option: full `owner/repo` — `github/app`

When enabled, newly created projects would default to the `owner/repo` display name. Ideally this could also be applied to existing projects. Manual project renaming should still override the derived name.

### Workflow impact

Anyone managing projects across multiple orgs/owners with overlapping repo names benefits. It removes ambiguity in the project list/sidebar, reduces the need for manual renaming, and makes it clear which org a session belongs to at a glance.

### Installation context

N/A — this is a Copilot app project-display/config request, not tied to a specific install scope.

### Additional context

The full repository (`owner/repo`) is already stored internally per project (`project_github_repo`); this request is about surfacing the owner in the default display name via a configurable option.

Start by tracing the project default display-name derivation and the existing project_github_repo value. Determine where global or per-project configuration and manual rename precedence belong, then verify behavior for new projects, existing projects if supported, and duplicate owner/repo names with focused tests.

Written by the indexing model from the issue text.