Fix: Coverage badge checkout ambiguity

[stephanprobst]

Problem

The CI build failed with:

fatal: 'coverage' could be both a local file and a tracking branch.
Please use -- (and optionally --no-guess) to disambiguate

This happens because the ./coverage/ directory exists from the coverage report generation, and git checkout coverage is ambiguous.

Solution

• Use git fetch origin coverage:coverage to create local branch directly
• Use git switch instead of git checkout (branch-only, no file ambiguity)
• Use git switch --orphan for cleaner orphan branch creation