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

CONVENTIONS.md

@@ -38,6 +38,7 @@ Files live in `includes/` or one level of subdirectory under `includes/`.
 3. Add the class name to `PR_Core_Migration_Runner::MIGRATIONS` array (index = version - 1).
 4. Bump `PR_CORE_TARGET_SCHEMA_VERSION` in `peptide-repo-core.php`.
 5. All migrations must be idempotent (safe to run twice).
+6. **Idempotency caveat (all-or-nothing skip):** migration 0004's skip guard requires *all* populated keys; a post with legitimately empty aliases will re-run on manual re-invoke — this is safe (write_meta only overwrites with non-empty values) but expected.
 
 ## How To: Add a New REST Endpoint