ci: update pinned upload-artifact action

[brunovicco]

## Summary
Align the report-upload workflow with the Claude harness by updating the
fully pinned `actions/upload-artifact` action from v4.6.2 to v7.0.1.

The workflow invocation remains unchanged, and the equivalent configuration
has already passed the Claude harness quality workflow.

## Change type
- [x] Refactoring or maintenance

## Validation
```text
git diff --check
Verified the action is pinned by the full v7.0.1 commit SHA.
GitHub Actions harness-quality workflow.

Impact review

• The change is focused and does not include unrelated files.
• Tests cover new or changed behavior, or the change is documentation-only.
• Template, profile, and plugin copies remain synchronized where behavior is duplicated.
• Service, library, and workspace impacts were considered.
• Security, secrets, permissions, and supply-chain impact were reviewed.
• Privacy, logging, telemetry, retention, and personal-data impact were reviewed.
• MCP trust, authentication, tool capabilities, prompt injection, and data egress were reviewed.
• Compatibility and migration impacts are documented when applicable.
• The complete relevant quality gates passed and evidence is listed above.
• CHANGELOG.md and VALIDATION.md were updated only when appropriate.