feat(agent): add executor evidence v2 contract

openspec/specs/chat-verifier-agent/spec.md

@@ -256,20 +256,24 @@ When Verifier returns `LOW_CONFID`, user-facing output SHALL clearly separate co
 ### Requirement: Executor SHALL output an evidence-attribution contract
 The Chat Executor SHALL produce a machine-checkable final output that separates confirmed claims from hypotheses, recommendations, and missing information.
 
-#### Scenario: Executor final output contains required top-level fields
-- **WHEN** Executor completes a Chat diagnosis step
-- **THEN** its final output SHALL contain `answer_version`, `diagnosis_summary`, `claims`, `hypotheses`, `recommended_actions`, `missing_info`, and `user_facing_answer`
+#### Scenario: Executor V2 final output contains only structured diagnostic fields
+- **WHEN** Executor completes a Chat diagnosis step under the V2 contract
+- **THEN** its final output SHALL contain `answer_version`, `claims`, `hypotheses`, `recommended_actions`, and `missing_info`
+- **AND** `answer_version` SHALL equal `executor_evidence_v2`
 - **AND** the output SHOULD be parseable as one JSON object without Markdown fences
+- **AND** the output SHALL NOT contain `diagnosis_summary`
+- **AND** the output SHALL NOT contain `user_facing_answer`
 
 #### Scenario: Confirmed claims carry evidence bindings
 - **WHEN** Executor emits an item under `claims`
 - **THEN** the item SHALL include `claim_id`, `claim_type`, `claim_text`, `support_level`, and `evidence_bindings`
-- **AND** `support_level` SHALL be one of `direct`, `indirect`, or `none`
-- **AND** claims with `support_level=direct` or `support_level=indirect` SHALL include at least one evidence binding
+- **AND** `support_level` SHALL be one of `direct` or `indirect`
+- **AND** `evidence_bindings` SHALL contain at least one evidence binding
 
 #### Scenario: Evidence bindings support multiple tool types
 - **WHEN** Executor binds evidence to a claim
-- **THEN** each binding SHALL include `source_type`, `source_id`, `tool_name`, `source_invocation_ids`, and `evidence_excerpt`
+- **THEN** each binding SHALL include `source_type`, `tool_name`, `source_invocation_ids`, and `evidence_excerpt`
+- **AND** the binding MAY include `source_id`
 - **AND** the binding SHALL be able to reference `lookup_knowledge`, `query_logs`, `query_metrics`, or other evidence-bearing tool traces
 - **AND** the binding SHALL NOT rely only on a RAG-specific `chunk_id`
 
@@ -286,10 +290,11 @@ The Chat Executor SHALL produce a machine-checkable final output that separates
 ### Requirement: User-facing Chat answers SHALL remain readable Chinese
 The system SHALL preserve a readable Chinese answer for normal Chat users even when Executor emits a machine-checkable contract.
 
-#### Scenario: User-facing answer is available
-- **WHEN** Executor emits structured output
-- **THEN** `user_facing_answer` SHALL be written in Chinese
-- **AND** it SHALL be consistent with the confirmed claims, hypotheses, recommended actions, and missing information in the same JSON object
+#### Scenario: V2 machine contract is not exposed as normal user answer
+- **WHEN** Executor emits `executor_evidence_v2`
+- **AND** Verifier returns `PASS`
+- **THEN** normal user output SHALL be rendered as readable Chinese from the structured contract or a safe fallback template
+- **AND** normal user output SHALL NOT be the raw Executor JSON object
 
 #### Scenario: Machine contract remains available for trace inspection
 - **WHEN** the Chat trace or verifier evaluation is inspected
@@ -309,3 +314,4 @@ The system SHALL tolerate malformed or absent structured Executor output without
 - **WHEN** Executor output parsing fails
 - **THEN** the verifier evaluation or trace snapshot SHALL make the parse failure visible
 - **AND** the failure SHALL NOT be silently treated as a successful evidence-attribution contract
+