Prompt templates as files in the repo

Prompts are behavior. In the prototype they were string constants scattered through the assistant package, edited ad hoc and impossible to diff against an incident.

Keep every prompt as a versioned file under backend/prompts/, loaded at startup. Changing a prompt is a code review.

Prompt changes became reviewable and bisectable. But a prompt fix rides the full deploy train — during the April carrier outage a one-line prompt patch took 40 minutes to ship. Superseded by the registry approach in ADR-007.