ci(deps): bump actions/download-artifact from 4 to 6 (#24)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/claude-plan-to-issues.yml

@@ -251,7 +251,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Download validated plan
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v6
         with:
           name: validated-plan
 

.github/workflows/dev-to-main.yml

@@ -171,7 +171,7 @@ jobs:
           pnpm-version: '9'
 
       - name: Download production build
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v6
         with:
           name: production-build