fix(resume): emit structured JSON for /agents --output-format json (#2987)

Resumed /agents --output-format json was returning a human-readable
text render wrapped in a JSON envelope field instead of the actual
structured agent list. The run_resume_command handler was calling
handle_agents_slash_command (text) for the json field instead of
handle_agents_slash_command_json.

Fix: use handle_agents_slash_command_json for the json outcome field,
matching the pattern already used by /skills and /plugins.

Test: extended resumed_inventory_commands_emit_structured_json_when_requested
to cover /agents, asserting kind=="agents", action=="list",
agents is an array, and count is a number (not a text render).

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -369,6 +369,34 @@ fn resumed_inventory_commands_emit_structured_json_when_requested() {
     assert_eq!(skills["action"], "list");
     assert!(skills["summary"]["total"].is_number());
     assert!(skills["skills"].is_array());
+
+    let agents = assert_json_command_with_env(
+        &root,
+        &[
+            "--output-format",
+            "json",
+            "--resume",
+            session_path.to_str().expect("utf8 session path"),
+            "/agents",
+        ],
+        &[
+            (
+                "CLAW_CONFIG_HOME",
+                config_home.to_str().expect("utf8 config home"),
+            ),
+            ("HOME", home.to_str().expect("utf8 home")),
+        ],
+    );
+    assert_eq!(agents["kind"], "agents");
+    assert_eq!(agents["action"], "list");
+    assert!(
+        agents["agents"].is_array(),
+        "agents field must be a JSON array"
+    );
+    assert!(
+        agents["count"].is_number(),
+        "count must be a number, not a text render"
+    );
 }
 
 #[test]