fix(missions): bind graph nodes to claws via attrs, not a dropped top-level key

`inject_node_agents` wrote the claw alias as a top-level `"agent"` key on
each graph node, but `cm_topology::Node` only deserializes `{id, role,
level, attrs}` — serde silently dropped it. `TurnRequest::agent` came back
`None` and every mission turn fell back to `ZEROCLAW_DEFAULT_AGENT`
(`scout`), running with scout's workspace and tools instead of the
mission's claws. The runtime trace confirms it: every turn logged
`"agent_alias":"scout"`.

That is why mission agents reported an "empty greenfield" workspace and
emitted artifacts inline instead of writing them: scout is jailed to
`/zeroclaw-data/.zeroclaw/agents/scout/workspace` and cannot see
`/mission/repo`. The per-mission provisioning and `workspace.path` pinning
shipped earlier were correct — they were just applied to agents that
nothing ever drove.

- bind into `node.attrs["agent"]` (top-level key kept for display/debug)
- extract the DB-free `apply_node_agents` and add a regression test that
  round-trips through the real `TopologyGraph` deserializer, which is the
  guard that was missing
- log loudly in `topology_exec::run_turn` when a node falls back to the
  default agent, instead of silently swapping in a different agent

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

crates/cm-api/src/topology_exec.rs

@@ -354,7 +354,19 @@ impl TurnExecutor for ZeroClawDriveExecutor {
             .map(str::trim)
             .filter(|a| !a.is_empty())
             .map(str::to_string)
-            .unwrap_or_else(|| self.alias_for(&req.role));
+            .unwrap_or_else(|| {
+                // Falling back here means the graph node was never bound to a
+                // claw, so the turn runs as the default agent with the DEFAULT
+                // agent's workspace and tools — not the mission's. That silently
+                // produced whole missions of unusable output, so say so loudly.
+                let fallback = self.alias_for(&req.role);
+                eprintln!(
+                    "topology_exec: node={} role={} has no bound agent — falling back to `{fallback}` \
+                     (its workspace/tools, NOT the mission's)",
+                    req.node_id, req.role,
+                );
+                fallback
+            });
         let prompt = Self::build_prompt(&req);
         self.drive(&alias, &prompt).await
     }