PRD: direct-prefix message machine scaffold

[cgrpa]

Parent

cgrpa/TheSexy6BotWorker#41

What to build

Build the initial transient per-message state machine and wire it into the direct-prefix happy path. The machine should handle one eligible incoming Discord message after trivial prefilters and orchestration have already run. It should use the prompt context helper, render a single response payload, send it to Discord, and run post-send bookkeeping.

The machine should stay focused on the message turn itself and should not own session lifecycle or trivial prefilters such as ping replies, bot-authored messages, or obvious command-like messages.

Acceptance criteria

• Eligible direct-prefix messages flow through the machine end to end.
• The machine starts after routing and session resolution have already happened.
• The machine uses the prompt context helper rather than rebuilding prompt data inline.
• The direct-prefix path renders and sends one response payload, then performs post-send bookkeeping on success.

Blocked by

• cgrpa/TheSexy6BotWorker#42