## Why
Parent-owned Multi-Agent V2 subagents must remain controlled by their parent,
but several app-server endpoints could still mutate them directly.
## What changed
- Centralize the direct-input ownership check and apply it to turn injection,
MCP calls, reviews, compaction, rollback and revert, shell and Guardian
actions, realtime requests, and goal mutations.
- Enforce the same policy for goal mutations on unloaded threads while keeping
goal reads and turn interruption available.
- Document the affected endpoint behavior and continue exposing
`canAcceptDirectInput: false` for parent-owned subagents.
## Testing
- Cover direct-input rejection for both legacy and paginated thread history.
- Cover goal mutation ownership checks for unloaded V2 subagents, V1
subagents, and non-subagent V2 threads.
GitOrigin-RevId: 791b7af98490aed5eab4e672b90661acad11ec23
rhan-oai
b6333bb1bb