Migrate Copilot PAT pool to shared workflow import (#10409)

* Update gh-aw to v0.71.5

* Migrate Copilot PAT pool to shared workflow import

Replaces the per-workflow inline `select-copilot-pat` action with the
shared `pat_pool.md` now usable with in gh-aw v0.71.5. The new pattern
fixes a bug where `COPILOT_PAT_0` was always selected because activation
`needs:` did not incorporate jobs referenced in `engine.env` expressions
(github/gh-aw#30232, fixed in v0.71.5).

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

* Add a workflow to monitor the PAT pool health

* Fix PAT pool output table

* Fix broken link in README

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

Author: jeffhandley