host: preserve journals, full traps and embedded agent output

Write new journal entries under logs/core and merge read-only legacy
history without moving SQLite/WAL files. Atomically spool failed batches,
retaining unspoolable entries for event-driven retries. Journal full trap
diagnostics before UTF-8-safe status display. Port session overflow archives
and followable line/byte excerpts to the still-shipped embedded agent.
Document retention/recovery and cover the contracts with regressions.
Include one rustfmt-only normalization in the existing SDK WASI test.

Card: SAU-143

cli/src/runtime.rs

@@ -212,7 +212,11 @@ fn short_trap(full: &str) -> String {
         .find_map(|l| l.find("wasm trap:").map(|i| l[i..].to_string()))
         .unwrap_or_else(|| full.lines().next().unwrap_or("trap").to_string());
     let mut line = line.trim().to_string();
-    line.truncate(300);
+    let mut end = line.len().min(300);
+    while !line.is_char_boundary(end) {
+        end -= 1;
+    }
+    line.truncate(end);
     line
 }
 
@@ -361,6 +365,7 @@ impl Runtime {
         });
 
         let (err_tx, err_rx) = mpsc::channel(1024);
+        let trap_log = log.clone();
         let stderr_task = tokio::spawn(async move {
             let mut reader = BufReader::new(host_err);
             let mut bytes = Vec::new();
@@ -400,6 +405,9 @@ impl Runtime {
                     Some(I32Exit(c)) => Exit::Code(*c),
                     None => {
                         let full = format!("{e:?}");
+                        if let Some((journal, pid, plugin)) = &trap_log {
+                            journal.write(pid, plugin, "core", &full);
+                        }
                         Exit::Trap(short_trap(&full))
                     }
                 },
@@ -416,3 +424,17 @@ impl Runtime {
         })
     }
 }
+
+#[cfg(test)]
+mod trap_text_tests {
+    use super::*;
+    #[test]
+    fn short_trap_display_is_utf8_safe() {
+        let full = format!("backtrace\nwasm trap: {}\nmore detail", "😀".repeat(1000));
+        let status = short_trap(&full);
+        assert!(status.len() <= 300);
+        assert!(status.starts_with("wasm trap:"));
+        assert_eq!(short_trap(""), "trap");
+        assert_eq!(short_trap("first line\nsecond"), "first line");
+    }
+}