Implement bounded Discord context loader for agent requests

[michaelmwu]

Parent: #265

Goal

Implement bounded Discord thread/channel context loading for agent requests.

Scope

• Add a backend context loader interface with deterministic source adapters.
• Implement a Discord current-thread/current-channel adapter.
• Support max-message, max-age, and max-token bounds.
• Return source-labeled snippets instead of anonymous raw text.
• Treat loaded messages as untrusted data.
• Do not implement global user-history search in the MVP.

Guardrails

• Only load messages the actor is allowed to see.
• Avoid loading private/restricted channels unless policy permits it.
• Include enough metadata for audit: channel id, message ids, loader, bounds, and token count.
• Do not store raw Discord message bodies indefinitely as part of this issue.

Acceptance criteria

• The agent can receive recent current-thread context when authorized.
• Loaded snippets are bounded and source-labeled.
• Private/restricted context is blocked without the right policy decision.
• Tests cover max bounds, source labeling, and denied access.