Fix GBK decode crashes on Windows by forcing UTF-8 everywhere

[Holmescalvin]

Summary

• subprocess.run(text=True) and Path.read_text()/write_text() without an explicit encoding fall back to the OS default codec, which is GBK on Chinese Windows.
• Reading/writing this project's UTF-8 source/data files (or capturing subprocess output containing non-ASCII bytes) then crashes with UnicodeDecodeError — e.g. running s10_system_prompt/code.py and reading README.md fails immediately.
• Forces encoding="utf-8" on every read_text/write_text call, and adds encoding="utf-8", errors="replace" to every subprocess.run(..., text=True, ...) call across the tutorial code.py files, agents/*.py, and the agent-builder skill scripts, so output stays readable instead of crashing the process even if a command emits unexpected bytes.

Test plan

• python -m py_compile on every changed file — all compile cleanly
• Ran s10_system_prompt/code.py end-to-end on Windows (Chinese locale/GBK) after the fix — starts and reads README.md/memory files without crashing
• Reproduced the original crash before the fix (UnicodeDecodeError / TypeError: unsupported operand type(s) for +: 'NoneType' and 'str') to confirm root cause

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