update: docs

[Abhijith Neil Abraham (abhijithneilabraham)]

Adds a docs/ folder with four focused reference documents for contributors and
self-hosters.

• agent.md — agent runtime internals: prepareAgentRun flow, system prompt
  sections, tool list, and the 3-layer context management system (pruning →
  memory flush → compaction) with key constants
• database.md — schema model reference explaining each field's purpose and the
  message type system
• integrations.md — Telegram bot setup (webhook registration, linking flow)
  and cron job execution mechanics, including Vercel Hobby plan limits
• api.md — full tRPC procedure reference and HTTP route table

Intentionally skips framework/tech-stack education (that lives in README.md
and CLAUDE.md) and focuses purely on product domain knowledge.