feat: add Claude auto-merge to upstream-sync workflow

- New merge-upstream job: creates branch, merges upstream, runs Claude
  agent mode to review, resolve conflicts, run tests, and create a PR
- Dynamic fork impact analysis: computes overlap between fork-modified
  files and upstream-changed files to assess conflict risk
- Claude reviews ALL upstream changes for fork compatibility (renamed
  imports, changed APIs, GitHub-only assumptions) not just conflicts
- Maintainability principle: minimize fork divergence, prefer adapters
- Remove ANTHROPIC_API_KEY (using OAuth token only)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/workflows/test-action.yml

@@ -34,8 +34,6 @@ jobs:
         claude_code_oauth_token: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
         github_token: ${{ secrets.FORGEJO_CLAUDE_TOKEN }}
         platform: forgejo
-      env:
-        ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
     - name: Verify execution output exists
       run: |-
         EXEC_FILE="${{ steps.claude.outputs.execution_file }}"