source/flag: command-line flags from a descriptor

[tdakkota]

Goal

A source/flag adapter, so a descriptor also produces command-line flags:

cfg, report, err := ConfigDescriptor.Resolve(
    yaml.File("config.yaml"),
    env.Current(env.Prefix("APP_")),
    flag.CommandLine(),          // highest precedence
)

Why it is not just another source

The three existing sources answer "what did this input say about field X". Flags
also have to answer the reverse — "what flags exist, and what does --help look
like" — which means registering a flag per field before parsing. The
figureout.Source interface (Load(ctx, *Model) (*Layer, error)) covers the
first half only, so this needs either a pre-parse step or a wider interface.

Scope

• flag.Name, flag.Short, flag.Skip, flag.Hidden, flag.Deprecated
  field options, with names relative to the declaring object as env and
  the tree sources already are (--server.listen-port, not
  --listen-port)
• Derived names from the canonical path: server.listenPort →
  --server-listen-port
• Help text from Doc, defaults from Default, allowed values from an
  enum constraint, and the value placeholder from the semantic type
• Booleans as presence-only flags (--debug, not --debug=true)
• Lists as repeated flags, delimited values, or both
• Unions: the discriminator is a flag, variant fields are flags alongside it
  (the same inline layout the other sources use)
• Origin reporting flag --server-listen-port so a bad value points at the
  flag, not at a file

Open questions

• Adapter target. The design lists flag.FlagSet, Cobra and Kong. Standard
  library first, or an interface with adapters from the start?
• Pre-parse registration. Add a Register(*Model) error step to the flag
  source specifically, or widen figureout.Source with an optional interface
  the resolver type-asserts?
• Required fields. A required field with no default currently errors when
  missing. With flags in the mix, does --help still work when required flags
  are absent? It has to.
• Merging. Repeated flags across one command line — MergeAppend within a
  single layer, or does the flag source flatten before producing its layer?