docs(template): close 9 RSR-template substitution gaps

Echidna was instantiated from rsr-template-repo; nine placeholder
strings never got replaced with echidna-specific values:

CODE_OF_CONDUCT.md
- L61, L111: echidna-conduct@example.org → j.d.a.jewell@open.ac.uk
- L48: "GitLab repositories" prose → "GitHub repository + GitLab CI mirror"

SECURITY.md
- L34: "public GitLab issues" → "public GitHub issues"
- L38, L205: security@echidna-project.org → j.d.a.jewell@open.ac.uk
- L134: same email substitution
- L206: "(when available)" PGP placeholder → fingerprint
  4A03639C1EB1F86C7F0C97A91835A14A2867091E + key fetch instructions
- L207: "@echidna-security (when available)" → @hyperpolymath

AUTHORS.md
- L47: "merge request on GitLab" → "pull request on GitHub"
- L99: malformed GitLab-style URL .../-/issues → /issues
- L100: contributors@echidna-project.org → maintainer email

Also adds SPDX-FileCopyrightText owner line to satisfy pre-commit.

Lane: prover-corpus-saturation. Adjacent to the saturation campaign;
small enough to roll into the same branch.

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

Author: hyperpolymath

AUTHORS.md

@@ -1,4 +1,5 @@
 <!--
+SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 SPDX-License-Identifier: MPL-2.0
 SPDX-FileCopyrightText: 2024-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 -->

CODE_OF_CONDUCT.md

@@ -1,4 +1,5 @@
 <!--
+SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 SPDX-License-Identifier: MPL-2.0
 SPDX-FileCopyrightText: 2024-2025 ECHIDNA Project Contributors
 -->
@@ -45,7 +46,7 @@ Project maintainers have the right and responsibility to remove, edit, or reject
 
 This Code of Conduct applies within all project spaces, including:
 
-- GitLab repositories (code, issues, merge requests, wiki)
+- GitHub repository (code, issues, pull requests, wiki) and any mirror (e.g. GitLab CI mirror)
 - Project communication channels (mailing lists, chat, forums)
 - Official project events (conferences, meetups, workshops)
 - Social media when representing the project

SECURITY.md

@@ -1,4 +1,5 @@
 <!--
+SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 SPDX-License-Identifier: MPL-2.0
 SPDX-FileCopyrightText: 2024-2025 ECHIDNA Project Contributors
 -->
@@ -148,6 +149,8 @@ Security advisories are published at:
   https://github.com/hyperpolymath/echidna/security/advisories
 - **GitHub Releases** (security patches are tagged accordingly):
   https://github.com/hyperpolymath/echidna/releases
+- **GitHub Releases atom feed**:
+  <https://github.com/hyperpolymath/echidna/releases.atom>
 
 ## Scope
 
@@ -232,7 +235,9 @@ For security-related questions or concerns:
   https://github.com/hyperpolymath/echidna/security/advisories/new
 - **Email**: j.d.a.jewell@open.ac.uk (maintainer)
 - **PGP key**: `4A03639C1EB1F86C7F0C97A91835A14A2867091E`
-  (https://keys.openpgp.org)
+  — fetch via `gpg --recv-keys 4A03639C1EB1F86C7F0C97A91835A14A2867091E`
+  or from <https://keys.openpgp.org/search?q=j.d.a.jewell@open.ac.uk>
+- **GitHub**: [@hyperpolymath](https://github.com/hyperpolymath)
 
 ## Acknowledgments