Introduces internal/layout package for centralized path construction,
migrates templates from name-based TEXT primary keys to UUID PKs with
team-scoped directories (WRENN_DIR/images/teams/{team_id}/{template_id}).
The built-in minimal template uses sentinel zero UUIDs. Proto messages
carry team_id + template_id alongside deprecated template name field.
Team deletion now cleans up template files across all hosts.
Implement the host agent as a Connect RPC server that orchestrates
sandbox creation, destruction, pause/resume, and command execution.
Includes sandbox manager with TTL-based reaper, network slot allocator,
rootfs cloning, hostagent proto definition with generated stubs, and
test/debug scripts. Fix Firecracker process lifetime bug where VM was
tied to HTTP request context instead of background context.
