docs: add solution design guideline to CLAUDE.md

Require root cause analysis and production scenario thinking
(high concurrency, multi-tenant, failure cascades) before designing
fixes or features. Prefer explicit config over runtime heuristics.

CLAUDE.md

@@ -93,6 +93,7 @@ Go conventions to follow:
 - **i18n strings:** When adding user-facing error messages, add key to `internal/i18n/keys.go` and translations to `catalog_en.go`, `catalog_vi.go`, `catalog_zh.go`. For UI strings, add to all locale JSON files in `ui/web/src/i18n/locales/{en,vi,zh}/`
 - **SQL safety:** When implementing or modifying SQL store code (`store/pg/*.go`), always verify: (1) All user inputs use parameterized queries (`$1, $2, ...`), never string concatenation — prevents SQL injection. (2) Queries are optimized — no N+1 queries, no unnecessary full table scans. (3) WHERE clauses, JOINs, and ORDER BY columns use existing indices — check migration files for available indexes
 - **DB query reuse:** Before adding a new DB query for key entities (teams, agents, sessions, users), check if the same data is already fetched earlier in the current flow/pipeline. Prefer passing resolved data through context, event payloads, or function params rather than re-querying. Duplicate queries waste DB resources and add latency
+- **Solution design:** When designing a fix or feature, identify the root cause first — don't just patch symptoms. Think through production scenarios (high concurrency, multi-tenant isolation, failure cascades, long-running sessions) to ensure the solution holds up. Prefer explicit configuration over runtime heuristics. Prefer the simplest solution that addresses the root cause directly
 
 ## Mobile UI/UX Rules