fix(infra): upload system prompt during clean deploy
The SYSTEM_PROMPT.md file is gitignored (contains personal info), so
infra/deploy.sh now uploads it to the server during initial deployment.
Also mounts the prompts directory in docker-compose.prod.yml so the
container can access the system prompt at runtime.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>