aspire agent init reports success without indicating selected skills are already installed

### Describe the bug

When `aspire agent init` installs selected skills whose files already match the embedded bundle, the installer correctly performs no writes. However, it also omits the installed-skills summary and only reports that agent environment configuration completed successfully.

This makes it appear that the skill selections were ignored, especially when running through a graphical client where the selection prompts and final command output are displayed separately.

The command should explicitly report that the selected skills were already installed and up to date.

### Expected Behavior

When every selected skill is unchanged, `aspire agent init` should display a message such as:

```text
Selected skills are already up to date.
Skills: aspire
Locations: .github/skills
```

This should be informational rather than an error. It should clearly distinguish a successful idempotent no-op from:

- No skills being selected.
- Skill installation being skipped.
- Bundle resolution or installation failing.

Mixed results should also be clear, for example when one selected skill is installed or updated while another is already current.

### Steps To Reproduce

1. Run:

```powershell
aspire agent init
```

2. Select a skill installation location and one or more skills.
3. Complete the installation.
4. Run `aspire agent init` again and select the same location and skills.
5. Observe that the command completes successfully but does not indicate that the selected skills were already installed and unchanged.

### Exceptions (if any)

None. The command exits successfully.

### Anything else?

The installer intentionally skips writes when file contents are unchanged. The missing feedback occurs because unchanged skills do not produce an updated-skill summary item, and the summary method returns without displaying anything when that collection is empty.

This behavior is present on the main branch. I did not find an existing issue specifically covering this UX gap.

Start at the `aspire agent init` command and the summary method that currently returns when the updated-skill collection is empty. Trace how unchanged and updated skills are collected, then make the output distinguish already-current skills, skipped installation, no selection, and failures, including mixed results. Add or update coverage for a repeated installation where all selected skills are unchanged and verify the command output.

Written by the indexing model from the issue text.