Adds the network half of the PRD 0001 egress topology: per-agent
--internal Docker networks with a slug-derived name and a numeric
conflict suffix that mirrors the container-name scheme in cli.sh.
Helpers cover create / attach / remove and are pipelock-agnostic, so
a future PRD can layer a different sidecar on top without entangling
the two concerns.
Refs: docs/prds/0001-per-agent-egress-proxy-via-pipelock.md
Assisted-by: Claude Code
