fix(issue-36): registry and wrapper accept --prompt-file; the wrapper hands the same path on

One argv string is capped at MAX_ARG_STRLEN (131,072 bytes on Linux), so a
prompt passed as --prompt could not reach the registry once it carried a
large diff. submit now takes --prompt-file and passes the same path to
`registry.py register --prompt-file`; the loop type's per-iteration update
goes by file too. --prompt keeps working unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-delegate-job/SKILL.md

@@ -44,6 +44,8 @@ multi-agent-mux-delegate-job submit \
   #   Liveness contract (Issue #18): the agent heartbeats every N s (automatic),
   #   the job is unresponsive after K x N s (--idle-timeout overrides), `started`
   #   must arrive within T s of the injection or the inject fails closed.
+  #   --prompt-file <path> replaces --prompt for long prompts (Issue #36): one
+  #   argv string is capped at 131,072 bytes on Linux. The loop always uses it.
 
 # 1b) Same, for an agent on another host (no herdr pane, no shared disk):
 #     the brief travels over the broker as a signed retained dispatch and a