Add ovsx publish step to release workflow and ship first Open VSX build

[deffenda]

Publish job in .github/workflows/ci.yml already pushes VSIX to VS Code Marketplace on tag pushes but does nothing for Open VSX. Adding a single ovsx step against the same artifact gives Cursor and Windsurf users a one-click install with zero ongoing maintenance, and the next tag push exercises the new path automatically.

{
  "scope": [
    "Edit .github/workflows/ci.yml: after 'Publish to Marketplace' step in publish job",
    "add 'Publish to Open VSX' step running 'npx ovsx publish $VSIX_PATH -p $OVSX_PAT' against same downloaded artifact",
    "with continue-on-error: false and env: OVSX_PAT: ${{ secrets.OVSX_PAT }}. Place before 'Create GitHub Release' so publish failure surfaces before release notes are cut. Update extension/CONTRIBUTING.md (or release runbook) noting tagged releases now publish to both marketplaces + OVSX_PAT must be valid. Cut patch tag (v1.2.1) once prerequisite ticket closed",
    "confirm workflow run shows Open VSX step green",
    "update issue #160 with open-vsx.org listing URL before closing it."
  ],
  "depends_on": [],
  "done_when": [
    "Extension installable from https://open-vsx.org/extension/deffenda/filemaker-data-api-tools at current package.json version AND most recent successful run of .github/workflows/ci.yml shows green 'Publish to Open VSX' step AND issue #160 closed with live listing URL in closing comment."
  ],
  "validation": {
    "local": [
      "yamllint .github/workflows/ci.yml"
    ],
    "review": []
  },
  "references": [],
  "surfaces": [],
  "artifact_expectations": [],
  "review_points": []
}