PRD: prompt context helper for per-message turns

[cgrpa]

Parent

cgrpa/TheSexy6BotWorker#41

What to build

Build a reusable helper that assembles the full prompt package for one eligible Discord message turn. The helper should gather the standard chat history, session history, reply-chain context, image attachment context, engagement-mode instructions, and version metadata into a mutable per-turn context object that the message pipeline can carry forward.

The helper should be the single place where the prompt for a turn is assembled, so later machine stages can rely on a ready-to-use context instead of rebuilding prompt data ad hoc.

Acceptance criteria

• The helper returns a mutable per-turn context object rather than just raw ChatHistory.
• The helper includes the expected session, mode, and metadata additions for a turn.
• The helper can be tested in isolation with deterministic inputs.

Blocked by

None - can start immediately