self-run: the task cut is its own function, tested at the boundary

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

crates/mind-body/src/self_run.rs

@@ -131,16 +131,10 @@ fn task_section(mind: &dyn MindPort, claims: &[PipelineRef]) -> String {
             _ => task.push_str(&format!("\n\nThe run claims `{}`, which could not be read as a follow-up: its task is unknown.", claim.id.0)),
         }
     }
-    if task.len() > TASK_LIMIT {
-        let mut end = TASK_LIMIT;
-        while !task.is_char_boundary(end) {
-            end -= 1;
-        }
-        task.truncate(end);
-        task.push_str("\n[task truncated]");
-    }
-    task
+    cut_at(task, TASK_LIMIT)
 }
+/// `text` cut at a character boundary to `limit` bytes, marked when cut.fn cut_at(mut text: String, limit: usize) -> String {    if text.len() > limit {        let mut end = limit;        while !text.is_char_boundary(end) {            end -= 1;        }        text.truncate(end);        text.push_str("
+[task truncated]");    }    text}
 
 /// What the child is told once, at the context ceiling.
 pub const FLUSH_MESSAGE: &str = "The context ceiling is reached. Admit to the mind what it lacks, close your run, and end.";

crates/mind-body/src/self_run/tests.rs

@@ -1385,3 +1385,12 @@ fn a_run_without_claims_gets_the_identity_lines_alone() {
     let ran = go(vec![Out(result_line())]);
     assert_eq!(ran.written()[0], start_message(&Slug("yggdrasil".into()), &run_ref("selfrun-a")));
 }
+
+#[test]
+fn the_cut_keeps_exactly_the_limit_whole_characters_and_marks_only_a_real_cut() {
+    assert_eq!(cut_at("abcd".into(), 4), "abcd");
+    assert_eq!(cut_at("abcde".into(), 4), "abcd\n[task truncated]");
+    // "\u{e9}" is two bytes: a limit inside one keeps the characters before it.
+    assert_eq!(cut_at("a\u{e9}\u{e9}".into(), 4), "a\u{e9}\n[task truncated]");
+    assert_eq!(cut_at("a\u{e9}\u{e9}".into(), 5), "a\u{e9}\u{e9}");
+}