ci: run full e2e across all oses

[numnx]

Summary

• Replace split Linux-full/native-smoke Playwright jobs with one shared full E2E matrix across Linux, macOS, and Windows.
• Run every Playwright purpose project (navigation, settings, projects, tasks, agents, config) on all three OSes from the same CI template.
• Convert orchestration into one cross-OS matrix: Linux validates the Docker-backed DAG, while macOS and Windows validate the same DAG through the Electron app.
• Keep the six-runner budget by capping orchestration at two concurrent shards and Playwright at three concurrent shards, with release-candidate packaging waiting for validation.
• Update workflow-health tests and docs so the cross-OS E2E/orchestration shape cannot silently regress.

Verification

• pnpm exec vitest run tests/backend/ci/workflow-health.test.ts tests/backend/docs/testing-docs-commands.test.ts
• python3 YAML parse for .github/workflows/*.yml
• pnpm run lint
• pnpm run check:docs-web