Hosted memory2 replay sharing

[xiaoyaoqilan]

#3158 is the requirements document. The implementation should be reviewed as three small PRs.

Why

A developer should be able to upload a memory2 recording and share one reference that DimOS can verify, cache, and replay. Today this requires manual file copying and local path setup.

Plan

1. Filesystem repository core — immutable SHA-256 objects, atomic writes, metadata validation, namespace isolation, and interrupted-write cleanup. No HTTP, UI, CLI, or cloud dependencies. Current draft: 535 lines across two files, 17 focused tests.
2. Transfer API and CLI — streamed/resumable HTTP upload and download plus dimos data upload/list/download.
3. memory2 replay integration — consistent SQLite snapshot upload, dimos-replay:// references, verified local cache, and the first Go2 replay consumer.

S3, WebRTC diagnostics, regional selection, CDN, continuous publishing, browser UI, ACLs, metrics, quotas, and PB-scale storage are separate follow-ups.

Questions

• Is this three-PR boundary reviewable?
• Should the repository service live in DimOS or in a separate package?