Complete recipe parameters and `arg` flags in Zsh

## Summary

Please extend the existing Zsh completion support so it can complete recipe-specific parameters, options, and flags from the resolved `justfile`.

`just` 1.55.1 already provides Zsh completion scripts via `just --completions zsh`, and the generated Zsh script delegates to `JUST_COMPLETE=zsh just`. This request is to make that existing completion path more recipe-aware after a recipe has been selected.

## Current behavior

Zsh completion can be installed with:

```console
$ mkdir -p ~/.zsh/completions
$ just --completions zsh > ~/.zsh/completions/_just
```

And then enabled in `.zshrc` with:

```zsh
fpath=(~/.zsh/completions $fpath)
autoload -U compinit
compinit
```

This provides completion for `just` itself and available recipes, but it does not complete the selected recipe's own parameters, options, and flags declared with `[arg(...)]`.

## Example

```just
[arg("target", help="Test target")]
[arg("verbose", long="verbose", value="--verbose", help="Use verbose output")]
test target verbose="":
cargo test {{verbose}} {{target}}
```

Expected completion behavior:

```console
$ just
test

$ just test --
--verbose Use verbose output
```

## Requested behavior

- Complete recipes from the resolved `justfile`, using the same justfile discovery behavior as a normal `just` invocation.
- After a recipe is selected, complete only the parameters, options, and flags that belong to that recipe.
- Complete long options and flags declared with `[arg(ARG, long="LONG")]`.
- Complete short options and flags declared with `[arg(ARG, short="S")]`.
- Include help text from `[arg(ARG, help="HELP")]` in Zsh completion descriptions when available.
- Support flags declared with `[arg(ARG, long="LONG", value=VALUE)]` or `[arg(ARG, short="S", value=VALUE)]`.
- Support options that take values, including repeatable variadic options.
- Avoid suggesting unrelated recipe names where recipe-specific arguments, options, or flags are expected.

## Motivation

This would make Zsh completion more useful for `justfile`s that expose recipes as small command-line interfaces with documented parameters, options, and flags.

Start by tracing the `just --completions zsh` generated script and its `JUST_COMPLETE=zsh just` completion path, then inspect how the resolved `justfile` and selected recipe are handled. Exercise the example `[arg(...)]` declarations and verify that recipe-specific long and short options, values, repeatable variadic options, and help descriptions appear only after the recipe is selected.

Written by the indexing model from the issue text.