feat: Agent-enhanced mindmap with incremental operations engine

[web4zn]

Summary

• Agent System — Web Worker + Vercel AI SDK tool calling, ReAct loop with 2 tools (readMindmap, generateMindmapOps)
• Streaming Chat — chat responses now stream token-by-token, decoupled from mindmap generation
• Incremental Operations — add_child / update / delete_leaf / add_root replace full tree replacement, ~70% token savings
• editedByUser Protection — per-operation level guarding, not post-hoc merge
• Smart Deduplication — chat history filtered for errors (401) and duplicate user messages
• Dual Mode — Phase 1 enhance mode complete (Agent runs in background after AI responds)
• Phase 2 — incremental ops engine, pattern-aware prompts, zero-LLM-call tool submission

Architecture

Chat (stream) → useConversation hook
                  ↓ onStreamComplete
Agent Worker → ReAct loop → readMindmap → generateMindmapOps → applyOperations → store

Files Changed

• 18 new files, 6 modified
• +1857 / -165 lines