feat(storage): add bounded storage usage report

[luoye520ww]

Problem

Storage governance needs a read-only, bounded usage report before cleanup or retention work can be safely added.

Scope

This PR adds only the storage usage scanner and its contract-level tests. It does not add UI, automatic cleanup, retention, or IPC wiring.

Changes

• Report threads, attachments, checkpoints, worktrees, logs, diagnostics, extensions, and models.
• Count bytes, files, directories, latest modification time, errors, and truncation per category.
• Use bounded per-category traversal with opendir and lstat; symlinks are never followed.
• Missing roots are reported as empty and malformed limits fail closed.

Validation

pm.cmd exec vitest run src/main/services/storage-usage-report.test.ts (3 passed)

pm.cmd run typecheck (passed)

pm.cmd --prefix kun run typecheck (passed)

pm.cmd run lint (passed)

pm.cmd run build (passed)

• git diff --check (passed)

Review

Completed functional, lifecycle, data integrity, security, cross-platform, performance, compatibility, and scope review. The scanner performs no writes or deletes.

Follow-ups

IPC/UI exposure and cleanup policy should be separate PRs.

[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Superseded by #944, which consolidates storage usage reporting, safe cleanup decisions, and data-retention validation into one reviewed PR.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.