Just completions read Makefile targets and ignore the project justfile

## Problem

On main `8d93f5b8574f3c7b8c30086d9c817bf7f5d4827d`, the environment reader dispatches both `make` and `just` to the Makefile parser. A project with only a justfile supplies no recipes; a project with both files supplies Makefile targets as `just` recipes. This creates incorrect environment-sourced terminal suggestions and omits the actual recipes.

[Reader dispatch and Makefile names](https://github.com/uttrflow/uttrflow-swift/blob/8d93f5b8574f3c7b8c30086d9c817bf7f5d4827d/Sources/UttrflowPredict/EnvironmentReading+System.swift#L122-L131).

The [just manual](https://just.systems/man/en/quick-start.html) specifies justfiles as the recipe source; its [listing interface](https://just.systems/man/en/listing-available-recipes.html) exposes recipe names.

## Reproduction

An isolated Swift test links the unchanged production Predict module and creates a temporary directory:

1. Write `justfile` containing a `verify-just` recipe with an echo command.
2. Call `SystemEnvironmentReader.values(of: .subcommand(of: "just"), in: directory)`.
3. Add `Makefile` containing a `verify-make` target with an echo command and call again.
4. Construct an `EnvironmentIndex(reader: reader)` and `EnvironmentSource(index: index)`. Request candidates for a terminal surface scoped to this directory, matching `just ver`; await `index.settle()` and request again.

Observed:

```text
Only justfile: nil
Both files: ["verify-make"]
Environment candidates: ["just verify-make"]
```

Expected: the `verify-just` recipe is available and `verify-make` is not supplied as a just recipe. Four expected-behavior assertions fail. The three existing `ProgramVerbsTests` pass, but test parsers independently of this dispatch.

This probe executes no recipes, downloads no tools or models, and does not claim an end-to-end UI acceptance result.

## Suggested fix and acceptance

Give `just` its own recipe discovery path, respecting its file discovery and recipe syntax. If using a command listing, bound and cancel the subprocess correctly (the existing timeout defect is #663).

Add reader-level regression coverage for a justfile-only project and a mixed Makefile/justfile project. Verify the real environment candidate path offers only the correct program's targets. Keep Makefile completion working.

## Related work

#751 asks for testable reader dispatch rules; this is a concrete behavior defect in that dispatch. #764 and PR #768 concern terminal validation, but do not correct this recipe source. Checked open and closed issues and the current open PRs before filing.

Start in Sources/UttrflowPredict/EnvironmentReading+System.swift at the reader dispatch and Makefile names linked in the issue, then inspect the existing recipe parsers and ProgramVerbsTests. Reproduce the justfile-only and mixed-file cases through SystemEnvironmentReader and EnvironmentSource. Done means reader-level regressions cover both cases, only the correct recipes appear in environment candidates, and Makefile completion still passes.

Written by the indexing model from the issue text.