Refactor matr to use a dedicated cli package

[euforic]

Summary

• extract the generated command runtime into github.com/euforic/matr/cli
• keep the root package as the launcher plus compatibility aliases for existing Matrfiles
• update generated runners and docs to point at the new cli runtime

Testing

• go test ./...
• go vet ./...
• golangci-lint run ./...

[Copilot]

Pull request overview

This PR refactors the command runtime implementation into a dedicated github.com/euforic/matr/cli package while keeping the root github.com/euforic/matr package as the CLI launcher and a compatibility layer via type aliases.

Changes:

• Moved the runtime implementation (Matr, Task, Flag, Invocation) into the new cli package and re-exported types/constructors from the root package for compatibility.
• Updated the generated runner template to instantiate and register tasks via cli.New() / cli.Task / cli.Flag.
• Updated and added tests to validate compatibility and ensure code generation uses the cli package; updated docs to describe the new package split.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
usage_test.go	Updates tests to use the new SetOutputs API instead of accessing unexported runtime fields.
template_test.go	Adds a regression test to ensure generated code imports/uses github.com/euforic/matr/cli.
template.go	Updates the runner template and flag type mapping to use the new cli runtime.
task.go	Converts root runtime types into compatibility aliases pointing at cli.
matr.go	Converts root Matr to a compatibility alias and forwards New() to cli.New().
compatibility_test.go	Adds an API-compatibility test ensuring root aliases remain assignable to cli types and Run behavior works.
cli/task.go	Introduces cli definitions for tasks/flags/invocations/handler types.
cli/matr.go	Introduces the cli runtime implementation (ported from the former root runtime).
README.md	Documents the new cli package and compatibility behavior in the root package.

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