chore(deps): update github artifact actions (major)

[Copilot]

Updates actions/upload-artifact from v4 to v5 and actions/download-artifact from v4 to v6.

Breaking Changes Analysis

upload-artifact v4→v5:

• Node v24.x support (no API changes)

download-artifact v4→v6:

• v5 fixed path behavior for single artifact downloads by ID (extracts to path/ instead of path/artifact-name/)
• v6 adds Node v24.x support

Impact

No changes required. This workflow uses name-based artifact downloads (name: build), unaffected by the v5 path behavior fix.

# Upload (unchanged)
- uses: actions/upload-artifact@v5
  with:
    name: build
    path: build

# Download (unchanged)
- uses: actions/download-artifact@v6
  with:
    name: build
    path: build

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