feat: initFilter allows looking other file types, ignore other than /_.

[vic]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a customizable initial filter for import trees, adds a .files shorthand to retrieve filtered files directly, and updates tests and docs to use these new features.

• Add initf parameter and initFilter API to override default file filter
• Implement .files attribute as a shortcut for leafs.result
• Update tests and README to cover initFilter and .files

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
default.nix	Introduce initf, compute nixFilter, wire up initFilter and .files, remove old leafs.result usage
checkmate.nix	Swap leafs.result to .files in tests, add tests for initFilter behavior
README.md	Document new .initFilter and .files APIs, update usage examples

Comments suppressed due to low confidence (2)

default.nix:6

• [nitpick] The parameter initf is ambiguous and doesn’t match the exported initFilter method; consider renaming it to initFilter for clarity and consistency.

      initf ? null,

default.nix:32

• The variable treeFiles is introduced but never used; consider removing it to clean up dead code.

          treeFiles = t: (t.withLib lib).files;