[Issacboy123]

Summary

This PR makes newly started subagent work durable in saved parent sessions. Before a child begins provider work, Diffex now saves enough information to find that child and understand how it was configured after a restart. Later status and history updates are also recorded, while in-memory and unsaved sessions continue to behave as temporary sessions.

Reasoning

The root-local factory and existing controller lifecycle are the narrowest ownership boundaries for choosing durable storage and recording state. Requiring the child header and queued roster before spawn acceptance prevents accepted work from becoming undiscoverable after restart. Later transitions remain best effort so a persistence failure does not make live controller state untruthful or halt already accepted work; strict admission failures instead restore the prior parent leaf.

Changes

• Made the root-local child-session factory choose parent-owned storage for persisted parents and retain in-memory storage elsewhere.
• Captured the child model, thinking level, built-in tools, and harness revision with each roster record.
• Gated spawn acceptance on a durable child header and queued full-roster snapshot before scheduling provider work.
• Persisted later status and child-leaf changes best effort and surfaced write failures through existing error and diagnostic paths.
• Persisted queued and running children as interrupted during graceful disposal, while still disposing after a snapshot failure.
• Added regressions for strict spawn ordering, admission rollback, lifecycle snapshots, graceful disposal, diagnostics, and unchanged in-memory, disabled, and unsaved behavior.

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