fix(tests): guard error_log/wp_json_encode stubs + document idempotency limitation

- tests/unit/test-migration-0004.php: wrap error_log() stub in
  function_exists() guard -- prevents PHP fatal redeclaration on
  PHP Lint (8.1/8.2/8.3) which runs php -l on all .php files.

- tests/bootstrap.php: update wp_json_encode stub to accept
  int $flags = 0 second arg (matches production call signature
  wp_json_encode($clean, JSON_UNESCAPED_UNICODE)); guard with
  function_exists().

- class-pr-core-migration-0004-backfill-peptide-meta.php: add
  code comment near idempotency check documenting the all-or-nothing
  limitation (post with legitimately empty aliases will re-run).

- CONVENTIONS.md: one-line note under migration conventions item 5
  documenting the idempotency caveat.

- CHANGELOG.md: add Fixed section under v0.6.0 for CI/test-stub fix.

Agent-Session: peptide-schema-backfill v0.6.0 P1 fix

tests/unit/test-migration-0004.php

@@ -71,9 +71,11 @@ function esc_html( string $val ): string {
 	return htmlspecialchars( $val, ENT_QUOTES, 'UTF-8' );
 }
 
-function error_log( string $msg ): bool {
-	// Suppress during tests.
-	return true;
+if ( ! function_exists( 'error_log' ) ) {
+	function error_log( string $msg ): bool {
+		// Suppress during tests.
+		return true;
+	}
 }
 
 // Load dependencies.