feat(storage): define data retention contract

[luoye520ww]

Problem

Kun has log-only retention settings, but storage cleanup also needs a consistent policy for threads, attachments, checkpoints, worktrees, diagnostics, extension logs, and model caches.

Scope

Adds a shared category allowlist and strict retention policy normalizer. Categories accept bounded day counts or null for keep-forever. Age checks revalidate untrusted policies and retain data on invalid/future timestamps.

Non-goals

No automatic deletion, disk scanner, backup integration, quota UI, or settings migration is included; those are separate PRs.

Tests

• npm.cmd exec vitest run src/shared/data-retention.test.ts (9 passed)
• npm.cmd run typecheck
• npm.cmd --prefix kun run typecheck
• npm.cmd run lint
• npm.cmd run build
• git diff --check

Review

Completed functional, data integrity, security, cross-platform, compatibility, and scope review. No package smoke is claimed because this PR is a shared contract-only change.

Issue

Part of #885

[luoye520ww]

The Windows packaged check is failing before this storage-only change reaches any behavior: the smoke waits for the packaged Extension guest target and times out. This is the same shared guest-target selector issue fixed independently in #915, so I am keeping that CI fix out of this storage contract PR to preserve scope. Once #915 is merged, this check should be rerun.

[luoye520ww]

Superseded by #944, which consolidates storage usage reporting, safe cleanup decisions, and data-retention validation into one reviewed PR. Closing this split PR to avoid duplicate review.

[luoye520ww]

Closed in favor of #944.