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/tests/plugins/probe/src/main.rs

@@ -376,6 +376,7 @@ fn dispatch(request: &Value, lines: &mut Lines) -> Option<Value> {
     let params = &request["params"];
     let result = match request["method"].as_str().unwrap() {
         "batch" => batch(lines),
+        "trap" => std::arch::wasm32::unreachable(),
         "nested_environment" => nested_environment(lines),
         "forget" => forget(lines),
         "forgotten" => {