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

agent/src/main.rs

@@ -94,19 +94,38 @@ impl Agent {
                 return Ok(final_text);
             }
             for (id, name, arguments) in calls {
-                let output = match serde_json::from_str(&arguments) {
-                    Ok(args) => tools::run(&name, &args)
+                let args = serde_json::from_str::<serde_json::Value>(&arguments);
+                let output = match &args {
+                    Ok(args) => tools::run(&name, args)
                         .await
                         .unwrap_or_else(|error| json!({"error":error})),
                     Err(error) => json!({"error":format!("arguments are not valid JSON: {error}")}),
                 };
-                append(
-                    &mut entries,
-                    Entry::ToolResult {
-                        id,
-                        output: tools::limit_output(output.to_string()),
-                    },
-                )?;
+                let output = if name == "read_file"
+                    && args
+                        .as_ref()
+                        .ok()
+                        .and_then(|a| a["path"].as_str())
+                        .is_some_and(|p| p.starts_with("/logs/"))
+                {
+                    output["content"]
+                        .as_str()
+                        .map(str::to_owned)
+                        .unwrap_or_else(|| output.to_string())
+                } else {
+                    output.to_string()
+                };
+                let tail = args
+                    .as_ref()
+                    .ok()
+                    .and_then(|a| a["method"].as_str())
+                    .is_some_and(|m| {
+                        [".exec", ".log", ".build", ".test"]
+                            .iter()
+                            .any(|suffix| m.ends_with(suffix))
+                    });
+                let output = tools::overflow::guard(output, "default", &id, tail);
+                append(&mut entries, Entry::ToolResult { id, output })?;
             }
         }
     }