This PR adds a new flagtype package that provides ready-made flag.Value implementations for common types that the stdlib flag package doesn't cover.

The supported types are:

• StringSlice (repeatable flags)
• Enum (restricted value set)
• StringMap (repeatable key=value pairs)
• URL (parsed and validated)
• Regexp (compiled pattern).

All types implement flag.Getter so they integrate with cli.GetFlag[T]. Storage is internal to each value -- no destination pointers needed.

f.Var(flagtype.StringSlice(), "tag", "add a tag (repeatable)")
f.Var(flagtype.Enum("json", "yaml", "table"), "format", "output format")

tags   := cli.GetFlag[[]string](s, "tag")
format := cli.GetFlag[string](s, "format")

Also includes a design doc at docs/design/001-flagtype-api.md covering the API decision and alternatives considered.