feat: support explicit can_spawn_agents, max_concurrent_agents, and max_agent_depth keys in graph.yaml

Graph agents previously derived can_spawn_agents from the presence of an agent node, so a pure-LLM-node graph could never spawn dynamic sub-agents. The three optional graph-level keys now override the derivation/defaults in AgentConfig::from_graph, enabling orchestrator graphs (can_spawn_agents: true with only llm nodes). Explicit false on a graph with agent nodes still runs those nodes; only dynamic spawning from llm nodes is disabled.

graph.example.yaml

@@ -38,12 +38,23 @@ reasoning_effort: null             # Default reasoning effort for `llm` nodes th
 
 max_concurrent_jobs: 5             # Max background jobs (`job__*` tools) running at once across the
                                    # whole graph run: every `llm` node (including parallel branches)
-                                   # draws from this one pool, so the budget is graph-wide — there is
+                                   # draws from this one pool, so the budget is graph-wide. There is
                                    # no per-node override. Jobs themselves are node-local: the node
                                    # that starts a job must collect or cancel it before it ends, and
                                    # anything left running at node exit is cancelled. Overrides the
                                    # global setting; 0 disables background jobs for this graph agent.
 
+can_spawn_agents: true             # Whether `llm` nodes may spawn dynamic sub-agents via the
+                                   # agent__* tools. Defaults to true iff the graph has an `agent`
+                                   # node; an explicit value overrides that. Set it true on a graph
+                                   # with only `llm` nodes to build an orchestrator graph. Setting
+                                   # it false on a graph with `agent` nodes still runs those nodes,
+                                   # only dynamic spawning from `llm` nodes is disabled.
+max_concurrent_agents: 4           # Max dynamically spawned sub-agents running at once. Explicit
+                                   # value overrides the default of 4.
+max_agent_depth: 3                 # Max agent nesting depth (agents spawning agents). Explicit
+                                   # value overrides the default of 3.
+
 global_tools:                      # Tool universe an `llm` node's `tools:` whitelist draws from
   - web_search_coyote.sh
   - fetch_url_via_curl.sh