test(jobs): add feature, hardening, and surface test matrix for background jobs

Covers the plan's T7 matrix: zero-diff invariants when jobs are off
(byte-identical tool lists and prompts, None-vs-Some select_functions),
validation hardening (shell/path-shaped/PATH-resolvable names, undeclared
MCP servers, non-whitelisted and context-filtered tools, mapping-tool
aliases, mid-batch tool-scope freshness), process lifecycle (grandchild
process-group kill, pgid clear after normal completion, panic skips the
completion notification), guardrail behavior (finished-job discard on
force-terminate, bounded inject-then-terminate iteration burn), surface
conformance (concrete_tool_names exclusion, toggle rejection, tools_info
listing, infra preservation under empty filters), supervisor swaps
(use_agent/exit_agent kill running jobs, child contexts cannot reach
parent job ids), and graph-node job lifecycle with deferred notification
drain.

src/config/agent.rs

@@ -1515,4 +1515,51 @@ nodes: {}
         assert_eq!(config.top_k, Some(7));
         assert_eq!(config.embedding_model.as_deref(), Some("some:model"));
     }
+
+    #[test]
+    fn interpolated_instructions_without_job_declarations_is_byte_identical_across_job_settings() {
+        let agent = |max_concurrent_jobs| {
+            Agent::test_new(AgentConfig {
+                instructions: "hi".to_string(),
+                max_concurrent_jobs,
+                ..AgentConfig::default()
+            })
+        };
+
+        let baseline = agent(None).interpolated_instructions();
+        assert!(
+            !baseline.contains(DEFAULT_JOB_INSTRUCTIONS),
+            "no job guidance may be injected without job__ declarations"
+        );
+        assert_eq!(baseline, agent(Some(0)).interpolated_instructions());
+        assert_eq!(baseline, agent(Some(7)).interpolated_instructions());
+
+        let mut with_unrelated = agent(None);
+        with_unrelated.functions.append_todo_functions();
+        assert_eq!(
+            baseline,
+            with_unrelated.interpolated_instructions(),
+            "job guidance injection must key strictly on the job__ prefix"
+        );
+    }
+
+    #[test]
+    fn interpolated_instructions_with_job_declarations_appends_job_guidance() {
+        let config = AgentConfig {
+            instructions: "hi".to_string(),
+            ..AgentConfig::default()
+        };
+        let baseline = Agent::test_new(config.clone()).interpolated_instructions();
+
+        let mut agent = Agent::test_new(config);
+        agent.functions.append_job_functions();
+        let output = agent.interpolated_instructions();
+
+        assert!(output.contains(DEFAULT_JOB_INSTRUCTIONS));
+        let expected = format!(
+            "hi\n{DEFAULT_JOB_INSTRUCTIONS}{}",
+            baseline.strip_prefix("hi").unwrap()
+        );
+        assert_eq!(output, expected);
+    }
 }