Marketplace-ready release: universal VSIX + simplified publish pipeline

[MelbourneDeveloper]

TLDR

Make Diffy marketplace-ready: collapse the 6-platform release pipeline into one sequential job that builds a single universal VSIX and publishes it to the VS Code Marketplace, plus marketplace metadata/docs and a fix for an accidental test-fixture gitlink.

Details

Release pipeline (.github/workflows/release.yml, ~-176 lines):

• Replaced the multi-job, 6-platform VSIX matrix (linux/darwin/win x64/arm64) and the Shipwright ci-gate/validate-manifest/stamp/verify jobs with one sequential job triggered on a v* tag: npm ci (npm cache) → set version from tag → build → vsce package (no --target) → vsce publish → attach the VSIX to the GitHub release.
• Diffy is pure TypeScript with no native binaries, so a single platform-agnostic VSIX serves every OS/arch — the per-platform matrix was pure overhead.
• Publishes via the org-level VSCODE_MARKETPLACE_PAT secret, with a guard that fails loudly (with instructions to add Diffy to the secret's allowed repositories) when the secret isn't accessible. Prerelease tags (containing -) are flagged on the GitHub release.

Manifest (shipwright.json): platforms: ["all"] and artifact: "vsix-fat" to match the universal build (was the 6-platform list + vsix-per-platform).

Marketplace metadata (package.json): added bugs, homepage, keywords, galleryBanner, and the Other category.

Docs: new CHANGELOG.md documenting the 0.1.0 command surface; README.md rewritten for marketplace users and trimmed of the contributor/Development (make-targets) section.

Fix (.gitignore): test-fixtures/repo-seed/workspace/ was tracked as an accidental gitlink (phantom submodule), leaving the working tree perpetually dirty as the E2E suite re-seeded it. It is a script-built throwaway repo, now ignored.

No runtime/source code changed — release infrastructure, manifest, and docs only. No breaking changes.

How Do The Automated Tests Prove It Works?

• make ci passed end-to-end (exit 0): ESLint + tsc --noEmit + shipwright:validate, then the fail-fast unit + @vscode/test-electron E2E suite with coverage 95.06% statements / 87.97% branches / 97.82% functions (above the ratcheted threshold), then the tsc build. With no source changes, the green suite proves no regression.
• npm run shipwright:validate confirms the new platforms: ["all"] / vsix-fat manifest is schema-valid against schemas/shipwright.schema.json.
• Verified the release path locally: npm run build && npx vsce package produces diffy-0.1.0.vsix with no TargetPlatform in extension.vsixmanifest (a true universal VSIX) that bundles shipwright.json.