BEAM-local physical workspace snapshots for isolated Subagents.

This module is the snapshot half of ADR 0011's isolated child workspace contract. It deliberately walks and copies with Elixir filesystem APIs instead of shelling out to git, find, or another host process. That keeps Subagent fanout aligned with ADR 0027: BEAM coordination can scale broadly, while host-boundary crossings stay scarce and explicit.

The first policy is intentionally conservative. It skips common dependency, build, cache, and Pixir runtime-state directories by basename at any depth, enforces copy limits, and returns string-keyed metadata/errors suitable for Tool results and Events.