fix(runtime): announce the runtime image once, not on every sweep tick

`MissionRuntimeProvisioner::from_env` is called per use — on every mission
launch and from the terminal-mission reaper sweep — so the line added in
cdc45bd would have printed on every tick forever. A log that repeats
itself is a log nobody reads, which would have cost exactly the
visibility the line was added to provide.

Co-Authored-By: Claude Opus 5 <[email protected]>

crates/cm-api/src/mission_runtime.rs

@@ -583,9 +583,13 @@ impl MissionRuntimeProvisioner {
             Ok(v) if !v.trim().is_empty() => (v, "CLAWMATES_RUNTIME_IMAGE"),
             _ => (DEFAULT_IMAGE.to_string(), "the built-in default"),
         };
-        // Said once at startup, because every mission on this host runs in it
+        // ONCE per process. `from_env` is called per use — on every launch, and
-        // and nothing else names it out loud.
+        // from the reaper sweep — so a bare eprintln here is a line every sweep
+        // tick forever, which is how a log stops being read at all.
+        static ANNOUNCED: std::sync::Once = std::sync::Once::new();
+        ANNOUNCED.call_once(|| {
             eprintln!("mission_runtime: per-mission runtime image = {image} (from {source})");
+        });
         Some(MissionRuntimeProvisioner { docker, image })
     }