Select notification targets by stable ID

Select notification targets by stable ID

The notify command selects a permanent target with --agent-id and
defaults to root when the flag is absent. Agent names, unknown IDs,
empty IDs, and repeated flags are refused instead of redirecting
external data.

The local socket keeps acceptance receipts private and reports target
conflicts without waking another agent. Command help and isolated tests
cover explicit root IDs, child delivery, absent names, and the existing
untrusted-data and next-turn boundaries.

Issue: 44d8703