fix(openclaw): default memory search to local FTS

[btc69m979y-dotcom]

Summary

• Generate OpenClaw memorySearch config even when embedding search is disabled.
• Use provider none, fallback none, disable vector storage, and keep trigram FTS for the default local keyword-search path.
• Add a gateway-startup migration for upgraded users whose existing memory index was built with an embedding provider: when the current main-agent config is FTS-only and the index meta is stale, run openclaw memory index --force --agent main before starting the gateway.
• Keep current embedding-enabled configurations untouched; the migration skips when the effective main-agent memory search config still uses an embedding provider.

Validation

• npm test -- openclawConfigSync.runtime
• npm test -- openclawMemoryIndexMigration
• npx eslint --ext ts,tsx --report-unused-disable-directives --max-warnings 0 src/main/libs/openclawConfigSync.ts src/main/libs/openclawConfigSync.runtime.test.ts
• npx eslint --ext ts,tsx --report-unused-disable-directives --max-warnings 0 src/main/libs/openclawMemoryIndexMigration.ts src/main/libs/openclawMemoryIndexMigration.test.ts src/main/libs/openclawEngineManager.ts
• npm run compile:electron
• git diff --check