docs: holistic AGENTS.md update — current source layout, architecture, docs index

- Fix stale package path: BackendStack21/kode → BackendStack21/odek
- Rewrite source layout with all current packages (narrate, redact, transport, mcp, mcpclient, sandbox, auth, display)
- Document parallel tool execution, batch approval gate, interaction modes
- Add 5-layer config priority (secrets.env layer 0)
- Document new subsystems: secret redaction, dynamic model discovery, Telegram bot
- Update binary size: ~11 MB → ~12 MB
- Add release checklist and documentation file index
- Refresh testing conventions and common pitfalls from recent dev
- Expand from 163 to 233 lines

Author: molty3000