Align default version file globs with documented coverage

[CasperKristiansson]

Summary

Update the action's built-in glob list so the default scan matches what the README promises: include .tool-versions, tox.ini, Pipfile, Conda environment.yml, Dockerfile.*, and nested **/*.python-version discovery.

Why

Marketplace users rely on the defaults. Today we advertise support for these files, but the default paths miss several of them. That causes the action to skip pinned versions silently.

What needs to happen

• Extend the default glob set with the missing patterns.
• Preserve performance by avoiding redundant traversals (e.g. prefer Dockerfile* patterns that cover both root and nested cases).
• Add fixture coverage that proves each newly-covered file type is detected.
• Mention the change in the changelog and README defaults table.

References

• Roadmap: hardening focus on aligning defaults with documentation.
• README “Feature overview” and “Default globs” sections.

Acceptance criteria

• Running the action with defaults touches all documented file types in fixtures.
• Tests cover each newly-detected file type.
• README and action documentation stay in sync.