[codex] Add memory query result cache

[saurabhhhcodes]

Summary

• add a TTL-backed in-memory cache for identical memory_query dispatches
• invalidate cached query results after memory_add, memory_update, and memory_delete in the same session
• expose queryCacheHits and queryCacheMisses through memory_access_analytics
• cover cache hits, invalidation, TTL override, and analytics stats with focused Bun tests

Closes #44

Validation

• npx --yes bun test src/mcp-server-cache.test.ts
• npx --yes bun test
• npx --yes bun run build
• npx --yes tsc --noEmit fails on existing unrelated src/tui.ts(50,3): 'api.command' is possibly 'undefined'