chore(ci): remove dormant push-email notification workflow (#86)

Removes `.github/workflows/push-email-notify.yml`.

Per the standing estate directive, push-email notification is retained
only on the three defensive-lab repos (`007`,
`robodog-defensive-systems-lab`, `defensive-multiplicity`). Tangle is
not one of them.

The workflow was already `disabled_manually` and skipped on every run,
so this is a no-op for CI behaviour — it just removes a dormant,
secret-consuming workflow from the roster.

Note: `Dogfood Gate` is currently red on all branches for an unrelated,
pre-existing reason — `hyperpolymath/a2ml-validate-action` and
`hyperpolymath/k9-validate-action` now resolve as "repository not found"
to the Actions runner (they passed on 2026-07-23). See
`dev-notes/tangle-sitrep-2026-07-27.md`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath