chore: remove unused workflows and actions

Remove truly unused code while preserving all tested workflows:

Deleted (971 lines):
- Workflows: vscode-nightly-release, vscode-promote-prerelease,
  test-vscode-package, test-vscode-workflows-integration
- Actions: check-ci-status, detect-packages, download-vsix-artifacts
- Config: package.json, package-lock.json, docs/ARCHITECTURE.md

Kept all functional workflows:
- vscode-release-explicit.yml (used in production)
- vscode-package.yml (tested)
- vscode-ci-template.yml (tested)
- vscode-publish-extensions.yml (tested)
- publish-vsix action (used by publish-extensions)
- npm-install-with-retries action (used by multiple workflows)

All inputs required by caller workflows are preserved.

Author: madhur310