chore: remove unused workflows (ado-automation, pre-commit, pull-request-checks) (#764)

Audited after the workflow-wipe restore (#763). Removing 3 workflows that have not done useful work in over a year:

* ado-automation.yml — 102 runs, all failure. Issue-to-ADO-user-story creation has been broken since at least mid-2025 (likely a stale ADO_PAT or area/iteration path). If we want this capability back, do it via an ADO-side webhook or the ado-msdata tooling rather than a curl-from-public-repo workflow.

* pre-commit.yml — all push/pull_request triggers commented out since 2025-05-09. Last actual triggered run >1 year ago.

* pull-request-checks.yml — same story; triggers commented out, the only runs since are skipped (workflow_dispatch event fails the pull_request guard). The size-check the workflow performs is not enforced elsewhere.

Kept: redirect-pull-requests.yml (auto-close, in active use), mirror-back.yml (new, needs separate debug), run-setup.yml (deferred — checking with infra owner).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brandom-msft