feat: add version_file input to read the kind version from a file

[somaz94]

Adds a version_file input that reads the kind version from a file, so the
version can live in a single source of truth (for example an asdf
.tool-versions file) instead of being hard-coded in the workflow.

• Supports the asdf .tool-versions layout (the kind entry) and plain
  version files.
• The version is used as-is (include the leading v, e.g. v0.31.0),
  consistent with the existing version input.
• When both are set, version_file takes precedence over version.

Validation

• Added a test-with-version-file job that writes kind v0.23.0 to a
  .tool-versions file and asserts the installed kind version is v0.23.0
  (the action default is v0.31.0, so the assertion proves the file drove
  the install).
• Unit-tested the resolver against: asdf multi-tool file, plain version file,
  comment/blank-line file, extra whitespace, CRLF endings, multi-token line,
  missing file, empty file.
• bash -n main.sh passes.

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This PR was prepared with AI assistance (Claude Code). All changes were
reviewed and tested by me before submission.