RFC 0009: Epic / Tracking issue for unified source identity

[rmanibus]

Context

Track completion and rollout for RFC 0009 (Unified Source Identity).

RFC: rfcs/0009-unified-source-identity.md

Goal

Use a consistent source identity model across sources so listing, grouping, and retention behave predictably.

Scope

• Source identity fields and formatting updates.
• Backup/list/policy behavior changes using unified identity semantics.
• Source implementations updated to populate stable identity.
• Docs updates.

Linked PR

• feat: unified source identity (RFC 0009) #85

[rmanibus]

Implementation PR: #85

[rmanibus]

Related PR: #76 (list grouping by source identity)