fix(chat): hybrid dedupe for optimistic rows with plugin-prefixed text

[grapeot]

Summary

• Root-caused the persistent ghost-row duplicate to oh-my-opencode prepending [analyze-mode] system prompt to user message text server-side, making the optimistic row's original text never match the persisted version
• Replaced exact text comparison with a hybrid strategy: drop the optimistic row when either (a) texts match exactly / server text ends with optimistic text (suffix containment), or (b) timestamps are within 60 s
• Added two new regression tests: plugin-prefix scenario and timestamp-only dedupe
• Updated working.md with diagnosis and fix