Port the lark-cli Flag.Input capability: a KindString flag may declare
Input sources ("file" for @path, "stdin" for -) and the framework
rewrites the explicit token into the payload content before
required/enum/constraint/Validate checks. @@value escapes to a literal
@value; a single stdin consumer per invocation is enforced; a leading
UTF-8 BOM is stripped. Shortcut.Flag gains the same declaration and the
adapter maps it through; LeafSpec inherits it via the LeafFlag alias.
玉澜
9a8a41a318