Record originating client/build metadata for synced writes

[Stvad]

Context

During the Matrix prefs investigation, the local DB showed the destructive event as source = sync with tx_id = null. That proves the connected client received the write from sync, but it does not identify which client, browser origin/profile, app build, or extension version produced the bad upload.

Problem

For replicated/background writes, the current local event trail answers "not this client" but not "which client/build did this." That makes cross-device sync bugs hard to attribute, especially when a stale local row or preview build writes a full properties_json payload.

Proposal

Add non-secret origin metadata to uploaded command/write batches and preserve it in local diagnostics after sync materialization.

Useful fields could include:

• stable per-install/client id
• browser/profile/origin identifier where available
• app build id/version/git SHA
• extension source block id and source hash for extension-authored writes, where applicable
• bridge/profile id for agent-authored writes, where applicable
• upload timestamp / local tx id correlation that survives through server echo

Acceptance criteria

• A future row_events.source = sync record can be traced to an origin client/build without reading local secret stores.
• Metadata is available in diagnostics/SQL for investigations, but does not expose tokens, localStorage secrets, or user content beyond existing event snapshots.
• The implementation covers normal app writes, plugin/effect writes, and agent bridge writes.
• Tests verify metadata is attached on upload and visible after sync materialization.

Notes

This is observability/debuggability work. It would not have prevented the Matrix prefs clobber, but it would have made the responsible client/build much easier to identify.