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/bootstrap.php

@@ -113,8 +113,10 @@ function sanitize_text_field( $value ): string {
 	return is_scalar( $value ) ? trim( (string) $value ) : '';
 }
 
-function wp_json_encode( $data ): string {
-	return json_encode( $data ) ?: '[]';
+if ( ! function_exists( 'wp_json_encode' ) ) {
+	function wp_json_encode( $data, int $flags = 0 ): string {
+		return json_encode( $data, $flags ) ?: '[]';
+	}
 }
 
 function absint( $value ): int {