`aspire agent init` does not detect portable GitHub Copilot AppImage installations

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

On Linux, `aspire agent init` cannot detect an App-only GitHub Copilot installation when the official AppImage is launched directly without desktop integration.

GitHub Copilot App detection currently relies on either:

- `AI_AGENT=github_copilot_app_agent`, available while running inside an App session; or
- a GitHub Copilot `.desktop` entry in an XDG applications directory.

A directly launched AppImage does not necessarily install a `.desktop` entry, and a normal terminal does not inherit the App runtime marker. In that case, if Copilot CLI is also absent, `aspire agent init` does not detect Copilot and skips the shared MCP, skills, and telemetry-hook configuration.

### Expected Behavior

App-only Linux users should have a supported way for `aspire agent init` to recognize or explicitly select their GitHub Copilot installation outside an active App session.

If the App creates a reliable persistent marker, Aspire should use it. Otherwise, consider an explicit configuration/override mechanism and document the portable-AppImage limitation rather than scanning arbitrary filesystem locations.

### Steps To Reproduce

1. Download and run the official GitHub Copilot AppImage on Linux without installing a `.desktop` entry.
2. Ensure the standalone Copilot CLI is not installed.
3. Open a normal terminal where `AI_AGENT` and `COPILOT_CLI` are unset.
4. Run `aspire agent init`.
5. Observe that GitHub Copilot App is not detected and shared Copilot configuration is skipped.

### Exceptions (if any)

N/A

### Aspire doctor output

N/A — this is an installation-discovery limitation rather than a missing Aspire prerequisite.

### Anything else?

This limitation was identified while testing #19820:

- Detection concern: https://github.com/microsoft/aspire/pull/19820#issuecomment-5503278500
- Reproduction report: https://github.com/microsoft/aspire/pull/19820#issuecomment-5503387837
- The PR was approved with this limitation accepted for follow-up: https://github.com/microsoft/aspire/pull/19820#pullrequestreview-5085256640

Users who also have Copilot CLI installed are unaffected because the shared Copilot configuration is applied through CLI detection.

Start at the `aspire agent init` entry point and trace the GitHub Copilot detection paths for `AI_AGENT`, `COPILOT_CLI`, and XDG `.desktop` entries. Review the detection discussion linked from #19820, then establish and test behavior for a directly launched AppImage when the CLI and runtime marker are absent; document the limitation if no reliable detection or override is possible.

Written by the indexing model from the issue text.