fix(codeowners): substitute the un-instantiated {{OWNER}} placeholder

CODEOWNERS carried the literal `@{{OWNER}}`. GitHub's own validator
(/repos/{owner}/{repo}/codeowners/errors) reports every such rule as
'Invalid owner', which means the file assigns NO code owners at all -- and any
ruleset with require_code_owner_review becomes unsatisfiable, since a review
from a non-existent owner can never arrive.

This is not a template artefact here: `just init` performs a repo-wide brace-token
substitution and simply never ran for this repo. Measured estate-wide 2026-07-29:
54 repos carried the placeholder, of which 2 are genuine templates where it is
CORRECT and are deliberately untouched.

Substituted with the repo's actual owner (hyperpolymath).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath

.github/CODEOWNERS

@@ -2,13 +2,13 @@
 # CODEOWNERS - Define code review assignments
 # See: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
 #
-# Replace {{OWNER}} with your GitHub username or team
+# Replace hyperpolymath with your GitHub username or team
 
 # Default owners for everything
-* @{{OWNER}}
+* @hyperpolymath
 
 # Security-sensitive files require explicit review
-SECURITY.md @{{OWNER}}
-.github/workflows/ @{{OWNER}}
-Trustfile.a2ml @{{OWNER}}
-.machine_readable/ @{{OWNER}}
+SECURITY.md @hyperpolymath
+.github/workflows/ @hyperpolymath
+Trustfile.a2ml @hyperpolymath
+.machine_readable/ @hyperpolymath