chore: scrub personal gmail from docs/configs (estate-wide sweep)

Replaced jonathan.jewell@gmail.com with j.d.a.jewell@open.ac.uk per
the hyperpolymath author-attribution rule (the gmail is banned;
j.d.a.jewell@open.ac.uk is the canonical non-Git-context address).

PLACEHOLDERS files got a generic user-old@example.com so they remain
useful as templates.

.mailmap files in this repo (if any) are intentionally left — they
are the legitimate git mechanism for remapping historical commits
authored under the old gmail to the correct open.ac.uk address.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

docs/LANDSCAPE.adoc

@@ -1,5 +1,5 @@
 = Feedback System Landscape Analysis
-Jonathan D.A. Jewell <jonathan.jewell@gmail.com>
+Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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