feat: add per-turn transparency receipts

[QueryPlanner]

Summary

Add a /trace or /why command that gives users a privacy-safe receipt for the
last agent turn: model, tools, duration, token/cost metadata when available, and
external services that received data. This should provide user-facing trust without
exposing hidden reasoning or sensitive content.

Priority

P2

Level of Effort

Medium (1 day)

Scope

• Define a typed per-turn receipt produced by the ADK runtime and callbacks.
• Store only the latest bounded receipt per conversation.
• Add Telegram command routing and a transport-neutral formatter.
• Record external destinations such as OpenRouter, Jina, Mem0/Qdrant, weather,
  Brave Search, and sandbox services when used.
• Exclude prompts, chain-of-thought, credentials, and raw sensitive tool payloads.

Passing Criteria

• /trace returns the last completed turn receipt for the current conversation.
• Receipts cannot be retrieved across Telegram chats or topic threads.
• Tests verify model, tool, duration, token, and external-destination fields.
• Tests verify secrets, prompt contents, hidden thoughts, and raw tool arguments are
  absent.
• Receipt storage is bounded and cleaned up with the session lifecycle.
• The feature remains compatible with the embedded observability dashboard tracked
  in issue Embedded DuckDB Observability Dashboard #100.

Sources

• Available ADK runtime turn boundary
• Current lifecycle callbacks
• Current Telegram command router
• Related dashboard issue #100