Migrate Task.hs to optparse-applicative Cli API

Task.hs still uses the old docopt-based Cli API but Cli.hs was migrated to optparse-applicative.

Error

Constructor `Cli.Plan' does not...
Not in scope: type `Cli.Arguments'
Not in scope: `Cli.has'

Fix needed

Migrate Task.hs to use the new optparse-applicative based API as documented in Omni/Cli.MIGRATION.md.