Add CI gate job for auto-merge support

[MrPointer]

Adds an Installer CI Status gate job to the installer CI workflow. This job:

• Runs with if: always() so it always reports a status
• Checks the results of build, test, and e2e-test
• Fails only if any of those jobs actually failed
• Passes if they succeeded, were skipped, or were cancelled

A matching require-installer-ci ruleset has been created on the repo requiring this check. This enables auto-merge:

• Installer PRs: CI runs, gate job waits for results, auto-merge proceeds on pass
• Dotfiles-only PRs: workflow is skipped entirely, check stays pending (not blocking in rulesets), auto-merge proceeds immediately