fix(runtime): use checked millis conversion for session timestamps

Resolves clippy::cast_possible_truncation in session.rs.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

rust/crates/runtime/src/session.rs

@@ -811,7 +811,7 @@ fn normalize_optional_string(value: Option<String>) -> Option<String> {
 fn current_time_millis() -> u64 {
     SystemTime::now()
         .duration_since(UNIX_EPOCH)
-        .map(|duration| duration.as_millis() as u64)
+        .map(|duration| u64::try_from(duration.as_millis()).unwrap_or(u64::MAX))
         .unwrap_or_default()
 }