fix(graph): auto-merge only schema-declared keys from llm output

state_updates::apply merged EVERY top-level key of an llm/agent node's JSON
output into graph state whenever the node had an output_schema, and
structured::extract never validated the output against that schema. Any
prompt text able to steer the model — including plan/diff text pasted from a
repo under review — could therefore plant an undeclared key (e.g.
verification_commands) into state and overwrite a caller-declared value
before a downstream script consumed it.

- apply() now takes the node's output_schema and merges only keys present in
  schema.properties; undeclared keys are dropped. No schema, or a schema
  without an object properties map, merges nothing.
- Nodes without output_schema are unchanged (None == the old false path);
  explicit state_updates are unchanged and still see the full output via
  {{output.<key>}}, which remains the author-controlled way to lift a key.
- Callers: llm.rs, agent.rs (x2), rag.rs (None).
- Sweep test bundled_graph_output_schemas_declare_every_key_relied_on: for
  every bundled graph, every consumed state key has a declared writer
  (initial_state, variable, state_updates target, map key, script-emitted key,
  or an output_schema property), and no schema node's field list names a key
  its schema omits — so this restriction cannot silently break a shipped
  graph. It passes with no graph.yaml changes.
- graph.example.yaml documents the declared-keys-only merge; the residual
  disclosed in 665fee9's README/docstring notes is now recorded as closed.

graph.example.yaml

@@ -213,8 +213,10 @@ nodes:
                                       # agent/graph/global default when omitted.
     skill_instructions: null          # Per-node skill-hint text override; uses the built-in default when omitted.
     output_schema:             # Optional JSON Schema. The output is parsed to JSON
-      type: object             # and its top-level object keys auto-merge into state
-      properties:              # (so `topic` / `needs_deep_dive` become {{topic}} etc).
+      type: object             # and only the keys declared under `properties` auto-merge
+      properties:              # into state (so `topic` / `needs_deep_dive` become {{topic}}
+                               # etc). Undeclared keys the model emits are dropped; lift
+                               # one explicitly via state_updates: `{{output.<key>}}`.
         topic: { type: string }
         needs_deep_dive: { type: boolean }
       required: [topic, needs_deep_dive]
@@ -284,8 +286,9 @@ nodes:
           type: array
           items: { type: string }
       required: [context, sources]
-    # When `output_schema` is set, top-level keys auto-merge into state, so
-    # `context` and `sources` are produced without needing `state_updates`.
+    # When `output_schema` is set, the keys declared under `properties`
+    # auto-merge into state, so `context` and `sources` are produced without
+    # needing `state_updates`. Keys the schema does not declare are dropped.
     next: synthesize           # Joins with retrieve at `synthesize`.
 
   # --- script node (the diamond's join; also dispatches) -----------------
@@ -369,7 +372,8 @@ nodes:
                                # and is retired when the chain ends. A message delivered
                                # after the chain's last flagged agent step is accepted
                                # but never read.
-    output_schema:             # Optional. Same extraction as llm nodes
+    output_schema:             # Optional. Same extraction as llm nodes: only the keys
+                               # declared under `properties` auto-merge into state.
       type: object
       properties:
         summary: { type: string }