Add SocketAuditor that sends audit logs to the Coder workspace agent
via a Unix socket. This enables boundary audit events to be forwarded
to coderd for centralized logging.

Features:
- Batching: 10 logs or 5 seconds, whichever comes first
- Wire format: length-prefixed protobuf (compatible with coder agent proto)
- Automatic reconnection on connection errors
- MultiAuditor to combine local logging with socket forwarding

Environment variables:
- CODER_BOUNDARY_LOG_SOCKET: path to agent's Unix socket
- CODER_WORKSPACE_ID: workspace UUID for log attribution

When both env vars are set, boundary sends logs to both the local
logger and the agent socket.

…mode

- Replace CODER_BOUNDARY_LOG_SOCKET env var with --audit-socket CLI flag
  (env: BOUNDARY_AUDIT_SOCKET) for configuring the socket path
- Remove workspace_id from proto (coderd has this info from agent auth)
- Generalize BoundaryLog proto with oneof resource to support future
  resource types (e.g., file operations)
- Rename proto file to logs.proto
- Add comment noting wire compatibility with coder's agent proto
- Add simple mode (--simple) using HTTP_PROXY instead of network namespaces
  for environments where jail/namespace features aren't available

Remove --audit-socket CLI flag and always use the well-known path.