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/executor.rs

@@ -660,6 +660,7 @@ mod tests {
                 state_updates: None,
                 output_schema: None,
                 timeout: None,
+                inputs: None,
                 teammates,
             }),
             next: None,