CI Quality Investigator fails on every merged pull request

## Description

The CI Quality Investigator listens for `pull_request: closed` events while using the `copilot-pat-pool` environment. GitHub runs these events under `refs/pull//merge`, but that environment permits only `main`.

Consequently, `pre_activation` fails before executing any steps:

> Branch "refs/pull//merge" is not allowed to deploy to copilot-pat-pool due to environment protection rules.

The `collect` job succeeds, but all PAT-backed downstream jobs are skipped and the workflow is reported as failed.

## Impact

This has affected every PR-triggered execution since the workflow was introduced on August 14, 2026:

- 106 PR-triggered runs
- 97 failures
- No successful PR-triggered runs
- Example: https://github.com/dotnet/sdk/actions/runs/33197625941
- First occurrence: https://github.com/dotnet/sdk/actions/runs/31824301915