BREAKING CHANGE: `#[completion]` functions must now take `ShellContext`
by value instead of `&ShellContext`. Reference parameters are reserved
for resource injection.

Remove the `pack!`, `pack_err!`, `pack_structural!`, and
`pack_err_structural!` macros, replacing all pipeline type definitions
with `#[derive(Grouped)]` and `#[derive(Grouped, Wrap)]` attributes.
This changes the generated struct shape from named-field structs with an
`inner` field to tuple structs accessed via `.0`, and removes the
auto-generated `name` and `info` fields from error types.

Replace buffer-based renderers with explicit RenderResult returns and
use cargo-styled output macros for consistency.

Add package enable/disable functionality with version matching
for pkg-enable, and fix help text to reflect implemented commands.