refactor: move gate-check to consuming repos

[madhur310]

Prevent inversion of control by having consuming repos implement gate-check before calling shared promote workflow.

Changes:

• Add vscode-find-nightly-candidate.yml (reusable workflow to find nightlies)
• Simplify vscode-promote-prerelease.yml (publishing-only):
  • Remove find-nightly-candidate job (callers use vscode-find-nightly-candidate.yml)
  • Remove quality-gate job (callers use check-ci-status action inline)
  • Remove min-tag-age-days and required-ci-checks inputs
  • Make release-tag required (callers provide it)

Pattern: Consuming repos now:

1. Call vscode-find-nightly-candidate.yml to find tag
2. Call check-ci-status action inline to validate CI (repo controls criteria)
3. Call vscode-promote-prerelease.yml to publish (publishing-only)

Benefits:

• Each repo controls its own promotion criteria
• Shared workflows provide tools, not decisions
• Gate-check visible in consuming repo's Actions UI