feat(harness): add chat application use case

src/main/resources/db/migration/V012__add_chat_release_contract.sql

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+-- V012: persist intent routing and only the bounded result eligible for PreviousTurn.
+
+ALTER TABLE diagnosis_run
+    ADD COLUMN intent VARCHAR(32) NULL COMMENT 'SYSTEM_CHAT/KNOWLEDGE_QUERY/DIAGNOSIS' AFTER agent_flow,
+    ADD COLUMN release_outcome VARCHAR(16) NULL COMMENT 'SUCCESS/FALLBACK/FAILED/CANCELLED' AFTER intent,
+    ADD COLUMN published_result JSON NULL COMMENT 'Bounded safe PreviousTurn source; no tool ids or raw evidence' AFTER answer,
+    ADD INDEX idx_diagnosis_run_previous_turn (
+        session_id, intent, release_outcome, created_at, id
+    );

src/main/resources/prompts/intent-router-prompt.md

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+Classify one unchanged user query into exactly one route.
+
+SYSTEM_CHAT: product capabilities, usage, greetings, or non-business small talk.
+KNOWLEDGE_QUERY: internal documentation, process, API, error code, configuration, or runbook questions.
+DIAGNOSIS: troubleshooting, root cause, current runtime state, or multi-source evidence analysis.
+
+The optional last_intent and last_user_query only help resolve follow-ups. A clearly new current topic wins.
+You have no tools, memory, or permission to rewrite the query.
+
+Return exactly one JSON object with no markdown or extra fields:
+{"intent":"SYSTEM_CHAT|KNOWLEDGE_QUERY|DIAGNOSIS"}