fix(followups): F2 review R1 - a failed env load is not cached as loaded

ensure_env_loaded() set _env_loaded before _load_dotenv() ran. With F2,
load_env_or_report() turns the EnvFileError into a return value, so a second
main() in the same process skipped the unreadable file and ran unconfigured
(codex R1: registry.main twice -> rc 1 then rc 0 with a job registered; a
repaired file was never read). The flag is now set only after a successful
load: a failure is retried on the next call, a success still loads once.
envfile parses the whole file before applying, so a failed load sets nothing.

MESSAGING.md: the one-line ERROR is the Python entry points'; the wrapper
adds its own "ERROR: cannot load <path>" line (codex, minor).

Test: in one process, registry.main twice on an undecodable file is rc 1
both times, prints the ERROR line twice and registers nothing; the repaired
file is applied on the next call; a later change is not re-read. At 95eabc6
it fails with rc [1, 0], one job registered and the value unset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

.agents/skills/lib_py/delegation/mqtt_common.py

@@ -122,8 +122,10 @@ def ensure_env_loaded(workspace_dir: Optional[str] = None) -> None:
     global _env_loaded
     if _env_loaded:
         return
-    _env_loaded = True
     _load_dotenv(workspace_dir)
+    # Only a load that succeeded is cached: after an EnvFileError the next
+    # call reads the file again (load_env_or_report returns, it does not exit).
+    _env_loaded = True
 
 
 def load_env_or_report() -> bool: