What Problem This Solves

Android still exposed global transcript-cache and command-outbox purge paths after authentication cleanup moved to gateway-scoped deletion. Those unused APIs, DAO queries, preference helpers, and tests described a second cleanup model that production no longer called.

Why This Change Was Made

Remove the obsolete global cleanup surface and retain the canonical per-gateway clearGatewayCache path used by reset and forget flows. This is a deletion-only cleanup with no runtime behavior change.

User Impact

No user-visible behavior changes. Android cache ownership now has one smaller, gateway-scoped cleanup path instead of retaining unused global purge machinery.

Evidence

• :app:ktlintCheck passed on Blacksmith Testbox tbx_01kwxrqn7ztayqr7v8c0wedhkc.
• Targeted :app:testThirdPartyDebugUnitTest passed for SecurePrefsTest, GatewayBootstrapAuthTest, both controller cache/outbox tests, and both Room cache/outbox tests.
• :app:lintPlayDebug :app:lintThirdPartyDebug passed, 60 tasks.
• pnpm check:changed passed on the Android-only branch; the final exact-head gate runs through the repository prepare-run Testbox flow.
• Codebase Memory MCP full graph refreshed after the final rebase: 278,239 nodes / 1,115,187 edges. Removed symbols return zero graph results; gateway-scoped cleanup remains indexed.
• Fresh Codex autoreview passed before commit and again on the committed branch with no accepted/actionable findings.
• git diff --check passed.

AI-assisted: yes. The implementation was inspected and validated by the author.