feat(#2591): hive-jobq Node lifecycle — started/finished timestamps + failure reason

Node gains started_at/finished_at (chrono DateTime<Utc>, serialized
RFC 3339 on the wire per hive_sh4re::wire_time) plus error (String).
Graph::set_state self-stamps started_at on the first Running transition
and finished_at on the first terminal one, via an internal now_utc()
clock (keeps settle/complete signatures stable). Outcome::Failed(String)
carries the failure reason, set on the terminal transition.

hive-c0re complete_node builds Outcome::Failed(msg); its node_rt
side-table stays i64 for now (double-write) until #2637 reads the Node.

Toward #2637: the jobq graph becomes the source of truth for per-node
lifecycle so the queue can be sent to the client as-is.

hive-jobq/Cargo.toml

@@ -7,6 +7,7 @@ version.workspace = true
 workspace = true
 
 [dependencies]
+chrono = { workspace = true }
 serde = { workspace = true }
 thiserror = { workspace = true }