Add agent context envelope and session state for Discord workflows

[michaelmwu]

Parent: #265

Goal

Add an agent context envelope and session state model so every Discord agent request carries stable source identifiers and can tie planning, confirmation, execution, memory lookup, and audit together through one operation_id.

Scope

• Add request fields for:
  • operation_id
  • Discord message_id
  • Discord channel_id
  • Discord thread_id
  • optional parent message id
  • response destination visibility
• Persist or pass session context for:
  • original natural-language request
  • proposed plan
  • confirmation state
  • tool results
  • context sources used
• Ensure the Discord bot forwards enough metadata for backend policy decisions.

Acceptance criteria

• Agent requests include stable Discord source identifiers.
• Planning, confirmation, execution, and audit share the same operation_id.
• The response destination is represented explicitly enough to prevent private context from being echoed into public channels.
• Tests cover context envelope serialization and validation.