Add Discord agent gateway and natural-language command routing

[michaelmwu]

Parent: #257

Goal

Add the first agent gateway path behind the Discord bot so users can issue natural-language English commands without putting orchestration logic directly inside a cog.

Scope

• Add a Discord cog or command entrypoint for agent-assisted commands.
• Resolve Discord user, guild, channel, and message context.
• Send structured requests to a backend agent/orchestrator endpoint.
• Render back either:
  • a read-only answer
  • a proposed frozen plan
  • a confirmation request
  • an execution result
  • a denial with a clear reason

Suggested paths

• apps/discord_bot/src/five08/discord_bot/cogs/agent.py
• apps/api/src/five08/backend/routes/agent.py
• shared request/response schemas under packages/shared/src/five08/agent/

Requirements

• Keep the Discord bot thin.
• Do not let the bot directly execute privileged service changes.
• Include message/channel/user metadata in every agent request.
• Preserve a stable operation ID across planning, confirmation, execution, and audit.
• Make the endpoint usable for a task-management MVP first.

Acceptance criteria

• A Discord user can submit an English task-management request.
• The bot can display the resulting plan or response.
• Write actions are not executed until the authorization/confirmation layer permits them.
• The implementation has focused tests for request parsing, context forwarding, and response rendering.