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/crates/cargo-valid/src/main.rs

@@ -92,6 +92,7 @@ mod tests {
             "/config",
             "/state",
             "/state/core/journal.sqlite",
+            "/logs/core/journal.sqlite",
         ] {
             assert!(!std::path::Path::new(path).exists(), "{path}");
         }