fix: skip fenced code blocks in body/posix-paths

[MaximeGaudin]

Summary

• body/posix-paths scanned every body line, so a Windows-style path inside a fenced example (e.g. a "Before:" line in a ```text block) was flagged as a real path, while body/imperative-instructions in the same file already tracks fences.
• The rule now skips lines inside fenced code blocks, using the same fence tracking the imperative rule uses.

Fixes #97

Test plan

• New regression test failed before the fix
• New regression test passes after the fix
• Local checks mirror CI and passed before push
• CI green on the PR