Data-handling policy: what AI tools should not be given access to

## Why
AI tools have different data-retention and training policies, and different deployments of the same tool differ (consumer-tier vs enterprise-tier API). The repo now has real secrets-handling code (ADR-0003, [#214](https://github.com/ChrisonSimtian/Fallout/pull/214) AES-GCM v2) and will accumulate more sensitive content — fixtures with tokens, encrypted test secrets, customer-shaped integration data.

Without an explicit policy, safe behaviour ("don't paste that into a chat box") relies on every maintainer remembering the right call every time. That doesn't scale.

## Action
- [ ] Inventory paths/patterns that should never be fed to AI tools:
- Encrypted secret files (whatever format ADR-0003 lands on)
- Test fixtures with real-looking tokens, account IDs, or PII-shaped data
- Anything under future `secrets/` or `dev-env/` directories
- The maintainer-only credential store from milestone #11 (Slack/Discord/Cloudflare recovery info)
- [ ] Document the list + rationale in a policy file (likely `docs/ai-data-policy.md`, cross-referenced from AGENTS.md)
- [ ] Add machine-readable ignore files where the convention exists — `.aiignore`, `.cursorignore`, etc. — so respecting harnesses filter automatically
- [ ] Document which AI-tool data-handling tiers the project officially supports (e.g. "Claude Code with API on the Anthropic console under the Fallout-build org" vs "personal consumer ChatGPT" — the former allowed for more)
- [ ] Discipline for adding to the list when new sensitive content appears

## Coordinate with
- AGENTS.md adoption issue — policy linked from there
- ADR-0003 secrets work — encrypted secret file paths come from there
- Milestone #11 org-ownership — once the org has its own AI subscriptions/keys, that becomes the recommended path

## Done when
- A `docs/ai-data-policy.md` (or equivalent) exists, is linked from AGENTS.md, and lists do-not-feed paths with rationale
- Machine-readable ignore files exist for tools that support them
- A maintainer discipline exists for adding new sensitive paths