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

MESSAGING.md

@@ -366,7 +366,7 @@ The subscriber acts as the central execution watchdog:
 * **`.mam.env` Resolution Hierarchy (`_load_dotenv`)**:
   Configuration files are resolved with strict precedence rules:
   1. **OS Environment Precedence**: Any variable already defined in `os.environ` is preserved and never overwritten by file-based configs.
-  2. **Explicit Override (`MAM_ENV_FILE`)**: If `MAM_ENV_FILE` is set, only that specific file is parsed. If the specified file does not exist, it is skipped silently and ambient search is refused (no fallback to unintended parent configs): the process runs on its environment and the defaults. If it exists but cannot be read or decoded, that is an error (`EnvFileError`): the wrapper and the Python entry points print one `ERROR: <path>: cannot read (<exception type>)` line and exit 1.
+  2. **Explicit Override (`MAM_ENV_FILE`)**: If `MAM_ENV_FILE` is set, only that specific file is parsed. If the specified file does not exist, it is skipped silently and ambient search is refused (no fallback to unintended parent configs): the process runs on its environment and the defaults. If it exists but cannot be read or decoded, that is an error (`EnvFileError`): the Python entry points print one `ERROR: <path>: cannot read (<exception type>)` line and exit 1; the wrapper prints that line followed by `ERROR: cannot load <path> (see the line above)` and exits 1.
   3. **Workspace Root Auto-Discovery**: If `MAM_ENV_FILE` is not set, the resolver searches candidate paths in order: `MAM_REAL_ROOT`, `WORKSPACE_ROOT`, upward directory walk searching for `.agents` or `.git` boundary markers, and `os.getcwd()`.
   4. **Public Broker Security Alert (B-17)**: If the final resolved host falls back to the public sandbox `broker.hivemq.com`, a prominent security warning is emitted.
 * **Broker Config Resolution (`broker_config_from_job`)**: