feat: add job summary with version update details to auto-update workflow

[Copilot]

The auto-update workflow ran silently — no visibility into what changed across channels per run.

Changes

• Capture old versions — new step reads each channel's version from versions.nix into env vars before update.sh runs
• Generate job summary — new step writes a $GITHUB_STEP_SUMMARY Markdown report after the update covering each channel (stable, staging, dev):
  • Version diff table (Old → New), or "no update" if unchanged
  • Release Notes — fetched from dotnet/aspire GitHub Releases if a matching tag exists (v{version} or {version})
  • Commit Diff — per-commit list + compare link from dotnet/aspire compare API if both old and new version tags resolve; reuses the tag format confirmed during release notes lookup to minimise API calls

All API calls degrade gracefully — sections are omitted when no data is found.

Example output

## Aspire CLI Version Update Summary

### Channel: `stable`
No update. Current version: `13.1.2-preview.1.26125.13`

### Channel: `staging`
| | Version |
| --- | --- |
| **Old** | `13.2.0-preview.1.26158.5` |
| **New** | `13.2.0-preview.1.26158.6` |

#### Release Notes
[View on GitHub](https://github.com/dotnet/aspire/releases/...)
...

#### Commit Diff
[View full diff on GitHub](https://github.com/dotnet/aspire/compare/...)
- [`abc1234`](https://github.com/...) fix: some bug fix

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