feat(graph): support max_concurrent_jobs at the graph level

Graph agents could only inherit the app-wide job budget; the agent-level
header in graph.yaml now accepts max_concurrent_jobs alongside
model/temperature, flowing through AgentConfig::from_graph into the
run-wide supervisor. Deliberately graph-wide, not per-node: jobs outlive
the node that started them.

src/graph/validator.rs

@@ -998,6 +998,7 @@ mod tests {
             temperature: None,
             top_p: None,
             reasoning_effort: None,
+            max_concurrent_jobs: None,
             global_tools: Vec::new(),
             mcp_servers: Vec::new(),
             skills_enabled: None,