Make plan approval gate only active in plan mode

- Add in_plan_mode flag to Agent struct
- Add set_plan_mode() and is_plan_mode() methods
- Gate check now only runs when in_plan_mode is true
- CLI calls set_plan_mode(true) on /plan command and EnterPlanMode
- CLI calls set_plan_mode(false) on approval and CTRL-D exit
- Update integration test to enable plan mode
- Fix test YAML to use Vec<Check> for negative/boundary checks

crates/g3-core/tests/mock_provider_integration_test.rs

@@ -906,6 +906,9 @@ async fn test_plan_approval_gate_blocks_unapproved_changes() {
     // Set the working directory to the temp git repo
     agent.set_working_dir(temp_path.to_string_lossy().to_string());
     
+    // Enable plan mode (required for the gate check to run)
+    agent.set_plan_mode(true);
+    
     // Create an unapproved plan for this session
     let mut plan = Plan::new("test-plan");
     plan.items.push(PlanItem {
@@ -915,8 +918,8 @@ async fn test_plan_approval_gate_blocks_unapproved_changes() {
         touches: vec!["src/test.rs".to_string()],
         checks: Checks {
             happy: Check::new("happy", "target"),
-            negative: Check::new("negative", "target"),
-            boundary: Check::new("boundary", "target"),
+            negative: vec![Check::new("negative", "target")],
+            boundary: vec![Check::new("boundary", "target")],
         },
         evidence: vec![],
         notes: None,

crates/g3-core/tests/stream_completion_characterization_test.rs

@@ -616,11 +616,11 @@ items:
         desc: Works correctly
         target: test::module
       negative:
-        desc: Handles errors
-        target: test::module
+        - desc: Handles errors
+          target: test::module
       boundary:
-        desc: Edge cases
-        target: test::module"#
+        - desc: Edge cases
+          target: test::module"#
             }),
         };
         let write_result = agent.execute_tool(&write_call).await.unwrap();

crates/g3-core/tests/tool_execution_roundtrip_test.rs

@@ -420,11 +420,11 @@ items:
         desc: Works
         target: test
       negative:
-        desc: Errors
-        target: test
+        - desc: Errors
+          target: test
       boundary:
-        desc: Edge
-        target: test"#
+        - desc: Edge
+          target: test"#
             }),
         );
         
@@ -477,8 +477,8 @@ items:
     touches: ["src/test.rs"]
     checks:
       happy: {desc: Works, target: test}
-      negative: {desc: Errors, target: test}
-      boundary: {desc: Edge, target: test}"#
+      negative: [{desc: Errors, target: test}]
+      boundary: [{desc: Edge, target: test}]"#
             }),
         );
         agent.execute_tool(&write_call).await.unwrap();