fix: invalid // comment headers break YAML/just parsing (use #)

The estate-standardization "Owner:" header was written with C-style // in
files that use # comments (.gitlab-ci.yml, Justfile), which is invalid and
breaks those parsers. Changed `// Owner:` -> `# Owner:`. (.adoc/.rs files are
left as-is: // is a valid comment there.)

https://claude.ai/code/session_017TXizM5c1Yd9HWf7Y15YH2

Author: claude

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: MPL-2.0
-// Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+# Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 # Primary CI/CD - GitLab is the source of truth
 
 stages:

Justfile

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: MPL-2.0
-// Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+# Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 # Justfile for tangle
 
 # Default recipe — list available commands