[Issacboy123]

Summary

This PR lays the storage groundwork for subagents to survive when a saved parent session is closed and reopened. It gives each parent a safe place for child histories and records which subagents belong to the currently selected session branch. It also validates saved data so one damaged or conflicting child record does not prevent other children from being recovered later.

Reasoning

Storing complete snapshots as reserved parent-session entries reuses existing JSONL branch semantics and lets the newest entry, including an empty reset barrier, fully mask older state. Keeping child files beneath a parent-owned namespace prevents them from appearing in normal session discovery, while independent record validation allows usable siblings to survive corrupt or conflicting data. The child-header durability seam remains internal because it exists only to support dependent work admission.

Changes

• Added the versioned diffex.subagents roster codec with strict field validation and additive-field tolerance.
• Added latest-selected-branch lookup, empty-roster masking, and isolation for malformed children, duplicate names, and shared child IDs.
• Added parent-owned child-session storage with UUIDv7 validation, path containment, link rejection, and child header, parent identity, and leaf verification.
• Added an internal synchronous child-header persistence helper.
• Added focused coverage for roster masking and validation, owned-path safety, listing exclusion, identity checks, leaf checks, and header durability.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/suite/subagent-persistence.test.ts test/suite/subagent-services.test.ts test/suite/subagent-registration.test.ts test/suite/agent-session-harness-version.test.ts test/session-manager/fork-branch.test.ts test/session-selector-path-delete.test.ts
• npm run check