fix(skills): hash byte-backed bundle files

[LeonSGP43]

Summary

• make bundle_content_hash() accept both text and byte-backed skill bundle files
• hash raw bytes directly while preserving the existing UTF-8 path for string content
• add a regression test covering byte-backed bundle content

Why

Fixes #13408. SkillBundle.files is already typed as str | bytes, but bundle_content_hash() always called .encode("utf-8"), which crashes on byte content during skill update checks.

Testing

• pytest -o addopts= tests/tools/test_skills_hub.py -k bundle_content_hash

[alt-glitch]

Likely duplicate of #13531 — same fix for bundle_content_hash() bytes crash (#13408). Also overlaps with #9925 and #3751.

[alt-glitch]

Duplicate of #13531.