refactor: extract ReActRunner/BaseAgent, add Zod validation, fix agent bugs

[web4zn]

Summary

• Extract ReActRunner from agent.worker.ts (335 to 145 lines) �� separates message routing from ReAct loop logic
• Add BaseAgent abstraction for future multi-agent support (with tests)
• Add Zod schema validation �� LLM output validated before applying, preventing data corruption (14 tests)
• Extract system prompt from useMindmapAgent.ts into dedicated system-prompt.ts
• Split tool definitions to agent-tools.def.ts

Bug Fixes

• Agent mode routing: handleSend and handleRegenerate now correctly check agentMode from store at call time
• applyOne matched flag: fixed variable scoping in add_child tree recursion
• Button nesting in MindMapPanel/ModelSelector
• Textarea forwardRef for auto-resize support
• Button forwardRef

Tests

• 91 tests passing (14 new: schema + BaseAgent)
• No regressions

Specs

• New capabilities: agent-core, agent-schema added to openspec/specs/
• Change archived