BREAKING CHANGE: Replace the `build`/`builds` feature system with
compile-time macro-driven generation. `gen_program!()` now automatically
invokes `build_comp!()` and `build_pathf!()` when the `comp`/`pathf`
features are enabled, eliminating the need for `build.rs` and
`[build-dependencies]` blocks.

This removes the `build` feature, the `mingling::build` module, and all
related build-time API functions. Completion scripts are now written to
`{target_directory}/mingling/` instead of `target/release/`. The
`mingling_cli` uses `build_comp!("mling")` for its custom binary name.

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.

Add dedicated error renderer for "No packages installed" and
return it early instead of rendering empty package list.

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

Add `pkg-show` to display installed packages and their versions,
removing its NOT_IMPL marker from help. Include internal commands
to load enabled package paths and completion scripts.