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

templates/rsr-metadata.ncl

@@ -10,7 +10,7 @@
     description | String,
     version | String | default = "0.1.0",
     author | String | default = "Jonathan D.A. Jewell",
-    email | String | default = "jonathan.jewell@gmail.com",
+    email | String | default = "j.d.a.jewell@open.ac.uk",
     license | String | default = "PMPL-1.0-or-later OR LicenseRef-Palimpsest-0.5",
     domain | String,
     tags | Array String | default = ["RSR", "Rhodium Standard"],