Enforce rulespec creation with plan_write for new plans

Solves the tautology problem where the LLM would write invariants after
implementation, making them match what was done rather than constrain it.

Changes:
- plan_write now accepts 'rulespec' parameter
- New plans REQUIRE rulespec (fails with helpful error if missing)
- Plan updates don't require rulespec (backward compatible)
- Rulespec is parsed, validated, and written atomically with plan
- Updated system prompt with clear examples for new vs update
- Updated tool definition schema
- Updated all affected tests

New flow: task → plan+rulespec → user reviews BOTH → approve → implement

crates/g3-core/tests/stream_completion_characterization_test.rs

@@ -621,6 +621,15 @@ items:
       boundary:
         - desc: Edge cases
           target: test::module"#
+                ,
+                "rulespec": r#"claims:
+  - name: test_feature
+    selector: test.done
+predicates:
+  - claim: test_feature
+    rule: exists
+    source: task_prompt
+    notes: Test invariant"#
             }),
         };
         let write_result = agent.execute_tool(&write_call).await.unwrap();

crates/g3-core/tests/tool_execution_roundtrip_test.rs

@@ -425,6 +425,14 @@ items:
       boundary:
         - desc: Edge
           target: test"#
+                ,
+                "rulespec": r#"claims:
+  - name: test_feature
+    selector: test.done
+predicates:
+  - claim: test_feature
+    rule: exists
+    source: task_prompt"#
             }),
         );
         
@@ -479,6 +487,14 @@ items:
       happy: {desc: Works, target: test}
       negative: [{desc: Errors, target: test}]
       boundary: [{desc: Edge, target: test}]"#
+                ,
+                "rulespec": r#"claims:
+  - name: approval_test
+    selector: test.approved
+predicates:
+  - claim: approval_test
+    rule: exists
+    source: task_prompt"#
             }),
         );
         agent.execute_tool(&write_call).await.unwrap();