Anthropic now requires limited data retention for models it designates
as covered models (Mythos-class models, such as Claude Fable 5): prompts
and outputs are retained for 30 days for trust and safety purposes, on
every platform where these models are offered, including platforms with
zero-data-retention agreements. See [Anthropic's public
policy](https://support.claude.com/en/articles/15425996-data-retention-practices-for-mythos-class-models).

Our AI privacy docs previously described zero data retention as applying
to all Zed-hosted models unconditionally. This PR updates them to
document the exception:

- `docs/src/ai/privacy-and-security.md`: Adds a "Provider Safety
Retention for Designated Models" section covering what is retained, by
whom, for how long, that no-training commitments still apply, and that
bringing your own key does not avoid retention for covered models.
Qualifies the intro, request-path table, and provider commitments table
accordingly.
- `docs/src/ai/ai-improvement.md`: Qualifies the zero-data-retention
statements and links to the new section.
- `docs/src/business/privacy.md`: Qualifies the zero-data-retention
statement and links to the new section.

Release Notes:

- N/A