Files Changed: persist explicit session lineage roots and typed edges

Acceptance Criteria

• Every newly created session has exactly one lineage root.
• Registered task and session-spawn paths from a full or partial parent create typed parent-child edges that resolve to the same root; a pre-epoch legacy parent follows the explicit non-aggregating legacy-parent rule.
• A fork creates a distinct root and never inherits mutation ownership or external evidence.
• Child archival and deletion preserve the root-permitted origin projection without retaining child-private state beyond policy.
• Pre-existing sessions resolve as legacy and are never reconstructed into a false lineage.
• A lineage query returns only the root and its registered descendants, never unrelated concurrent sessions.

Testing Decisions

• Extend session lifecycle coverage for root creation, typed task edges, explicit session-spawn edges, forks, archival, deletion, and legacy migration.
• Add isolation cases with concurrent roots sharing a project and worktree.

Key Decisions

• Lineage is an explicit persisted relation, not an inference from transcript metadata or directory overlap.
• Session forks remain independent roots even when copied transcript content references an ancestor.

Constraints & Invariants

• Root ownership does not grant child access to another session's private evidence.
• Child removal cannot leave a dangling origin reference in a retained root receipt.

Prior Art

• Parent Files Changed: full session-lineage mutation ledger #972
• Existing task-parent and explicit session-spawn relationships

Deliberation Resolution

• LineageMode is one of legacy, partial, or full. A session created before capability rollout stays legacy; its first opted-in tracked mutation opens one partial epoch at a recorded boundary time.
• A registered task or session spawn from a full or partial parent inherits that parent's root and mode. A spawn from a legacy parent before a partial epoch starts a distinct full root and retains only non-aggregating legacy_parent provenance.
• Once a legacy session opens its partial epoch, it is a partial parent; every later registered spawn inherits that partial root and is labelled partial.
• task_spawn and session_spawn are disjoint edge kinds. A fork has no lineage edge and always starts a distinct full root.
• A retained origin projection contains only origin session ID, display title, edge kind, and deletion timestamp. Child-private context, evidence, and transcript content never survive child deletion through the root ledger.
• Default lineage queries return active descendants. Files Changed requests retained deleted-origin projections only when rendering existing receipt history, never as navigable child sessions.

Source

Part of #972.