feat(agent): add composer final answer

devflow/projects/2026-07-08-executor-composer-final-answer/brief.md

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+# Executor Composer Final Answer
+
+## Background
+
+Stages one to three moved the Chat diagnosis chain to structured Executor output, deterministic Gatekeeper validation, and Verifier `claim_checks`.
+
+Before this stage, `ChatService` still owned final answer rendering. PASS paths could use a temporary V2 renderer, while LOW_CONFID and REJECT paths used templates. That left final user-facing expression too close to Executor material and made it harder to prove that only Verifier-allowed claims reached the user.
+
+## Goal
+
+Add a Composer expression layer after Verifier:
+
+```text
+chat_planner
+  -> chat_executor
+  -> VerifierInputHook + Gatekeeper
+  -> chat_verifier
+  -> chat_composer
+  -> final answer
+```
+
+Composer produces user-facing answers from filtered material only:
+
+- `allowed_claims`
+- `allowed_hypotheses`
+- `missing_info`
+- `recommended_actions`
+- `rationale`
+
+## Scope
+
+- Added `chat-composer-prompt.md`.
+- Added `chat_composer` Agent construction in `ChatService`.
+- Added Composer input filtering from Verifier decision and Executor structured output.
+- Replaced PASS temporary V2 renderer usage with Composer-or-safe-fallback rendering.
+- Routed LOW_CONFID and REJECT final answers through Composer when Verifier output is valid.
+- Added deterministic fallback for malformed Composer output.
+- Persisted `composer_output` under `diagnosis_session.self_evaluation.verifier_evaluation`.
+- Updated sequential workflow tests.
+
+## Non-Goals
+
+- No Planner changes.
+- No Executor retry changes.
+- No Gatekeeper rule expansion.
+- No Verifier classification expansion.
+- No database schema migration.
+- No stage-five eval fixture expansion.
+
+## OpenSpec
+
+- Active change before archive: `openspec/changes/executor-composer-final-answer`
+- Capabilities: `chat-composer-agent`, `chat-verifier-agent`
+- Scale: standard