feat(graph): seed a child graph agent's state from an agent node's inputs map

Agent nodes gain an optional `inputs:` map of templates resolved against
the parent state and overlaid on the child graph agent's initial_state
before its executor starts. A lone {{key}} keeps the state value's JSON
type; mixed text renders to a string; initial_prompt is still owned by the
dispatcher. run_child_agent and run_active_agent_graph keep their
signatures as wrappers over the new *_with_inputs variants, and
run_agent_for_graph rejects inputs aimed at a config-only agent or
containing initial_prompt.

Design: plans/map-subgraph-branches-design.md §3.12 (C18), task T5.

src/graph/mod.rs

@@ -16,7 +16,9 @@ pub mod types;
 pub mod user_interaction;
 pub mod validator;
 
-pub use dispatch::{active_agent_graph_name, run_active_agent_graph};
+pub use dispatch::{
+    active_agent_graph_name, run_active_agent_graph, run_active_agent_graph_with_inputs,
+};
 pub use executor::GraphExecutor;
 pub use parser::{GraphParser, agent_has_graph};
 use serde_json::Value;