Review retention policy, privacy messaging, and cleanup controls

[acgxv]

Context: History retention defaults to indefinite storage pending a privacy and cleanup review.

Decisions to make:

• Data retention policy and defaults.
• User-facing privacy messaging.
• Cleanup controls (manual delete, auto-expiry, per-app rules).

Reference: docs/spec/ui/app_experience.md

Acceptance:

• Update the spec with retention and cleanup behavior.