Update all references across docs, examples, and configuration files to
use the shorter `extras` feature name.

Add a new `#[command]` attribute macro (feature-gated behind
`extra_macros`) that converts a plain function with a `Vec<String>`
parameter into a fully wired Mingling command. The macro generates a
`dispatcher!` call, a `#[chain]` wrapper, and a hidden module
re-exporting
all generated types.

Also add `CommandPattern` to the pathf pattern analyzer to recognize
`#[command]`-annotated functions and track their generated hidden
modules.

pipeline error routing

Implement `RenderResult::append_other()` to merge buffered content and
immediate output behavior from one result into another. Add the
`r_append!` macro with both explicit buffer (`dst, src`) and implicit
buffer (`src`) usage forms, re-exported from `mingling::prelude`.