Refactor AI pipeline and implement History V3 with thread support

[sleroq]

Summary

• Implement History V3 with full thread support for multi-branch conversation tracking
• Optimize chat history loading and storage with structured metadata blocks (slusha_meta)
• Refactor AI chat pipeline into modular context/generation flow
• Add configurable ai.replyMethod (json_actions or plain_text_reactions) with fallback retries across full context, shorter history, and reduced context without bot notes/memory
• Add prompt customization (toolPrompt, stepPrompt) and metadata-based target reference parsing
• Refactor text splitting to use blank lines for better content segmentation
• Remove deprecated useJsonResponses config option; rename chat reactions config fields for clarity

Key Changes

• History V3: New thread-aware history format with parent/child message relationships
• Metadata Blocks: Structured slusha_meta tags for message attribution and context
• AI Fallbacks: Multi-tier retry system when AI responses fail or timeout
• Plain Text Reactions: Support for send_chat_reactions tool via plain text for reliability

Verification

• deno check --allow-import main.ts
• deno lint
• AI_TOKEN="test" deno test -A
• scripts/build.bash
• deno task check (widget)
• deno task lint (widget)
• deno task build (widget)