fix(skills): read skill markdown as UTF-8

[JieZeng777]

Why

On Windows systems whose default locale is not UTF-8, the skill creator validator reads SKILL.md with the platform default encoding. Localized UTF-8 skills can therefore fail validation with UnicodeDecodeError, even though the skill file itself is valid.

What changed

• Read SKILL.md explicitly as UTF-8 in the skill creator validator.
• Add a portable regression test that simulates a non-UTF-8 Windows default code page.
• Document the UTF-8 contract for skill text resources and validation utilities.

Surface area

• Frontend UI — page / component / setting / interaction under frontend/
• Backend API — endpoint / SSE event / request-response shape under backend/app
• Agents / LangGraph — agent node, graph wiring, langgraph.json, or prompt change
• Sandbox — docker/ or sandboxed execution
• Skills — change under skills/
• Dependencies — new/upgraded entry in backend/pyproject.toml or frontend/package.json
• Default behavior change — changes existing behavior without the user opting in
• Docs / tests / CI only — no runtime behavior change

Screenshots / Recording

Not applicable; this change has no UI surface.

Bug fix verification

• Test path that reproduces the bug: tests/skills/test_skill_creator_quick_validate.py
• Did it go red on main and green on this branch? Yes. Before the fix, the regression test raised the simulated GBK UnicodeDecodeError; after the fix, it passes.

Validation

• python -m pytest tests/skills -q — 52 passed
• python -m ruff check --config backend/ruff.toml tests/skills/test_skill_creator_quick_validate.py — passed
• python -m ruff format --check --config backend/ruff.toml tests/skills/test_skill_creator_quick_validate.py — passed
• python skills/public/skill-creator/scripts/quick_validate.py skills/public/skill-creator — Skill is valid

AI assistance

Tool(s) used: Codex

How you used it: Codex helped inspect the Windows locale failure, write the regression test, implement the minimal encoding fix, update documentation, and run validation. I reviewed and understand the resulting change.

• I've read and understand every line of this change and take responsibility for it — it's not unreviewed AI output.

[CLAassistant]

All committers have signed the CLA.

[WillemJiang]

@JieZeng777 thanks for your contribution. Please click the CLA assitant button to sign the CLA first.

[JieZeng777]

@WillemJiang Thanks! I’ve signed the CLA, and the license/cla check is now passing.

I also investigated the failing Skill Review CI. The four reported errors come from existing files in the skill-creator package and are unrelated to this PR’s UTF-8 change. This PR only changes quick_validate.py, but the CI reviews the entire package. Would you prefer that these pre-existing findings be handled separately, or should I include a narrowly scoped cleanup in this PR?