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 `mling update` to fetch the latest GitHub Actions artifact for the
current platform, stage it as a tarball in the data directory, and apply
it on the next wrapper invocation. The wrapper extracts the staged
package over the installation directory while skipping the running
executable, then removes the staged file.

Add a new `cfg` command to view and edit configuration
items stored as key-value pairs in a JSON file.

Move the global help and completion handlers from the CLI binary
into the library crate so they can be reused by other entry points.

Move the CLI binary from `src/main.rs` to `src/bin/cli.rs`
and move shared program generation into a new lib.rs module.