agent_sdk: write sandbox files as utf-8, not the locale default

[amburger66]

What

setup_sandbox_directory wrote CLAUDE.md, .claude/validate_sandbox.py, .claude/settings.json and notes.md with a bare Path.write_text(), which encodes using the locale's preferred encoding. All four now pass encoding="utf-8" explicitly.

Why

VALIDATE_SANDBOX_SCRIPT contains an em dash at index 3026, so on a C/POSIX locale the write raised:

'ascii' codec can't encode character '—' in position 3026: ordinal not in range(128)

Sandbox setup runs once per agent query, so this was not one unlucky query — every query and every final-submission nudge failed the same way, and the run ended with:

Bilevel solve failed: no captured plan after 0 completed agent queries.
Tasks solved: 0 / 1

The cause is invisible from the logs: agent_model_based_approach catches the exception per query and logs only the message, so the failure reads like an agent/planning problem rather than a file write. It was found while bringing up pybullet_domino_real on real hardware, where it blocked the run entirely.

The system-prompt write further down the same function already passed encoding="utf-8"; this makes the rest consistent with it.

Verification

Ran setup_sandbox_directory directly under LC_ALL=C LANG=C PYTHONCOERCECLOCALE=0 PYTHONUTF8=0 (preferred encoding ANSI_X3.4-1968):

• pristine origin/master — raises the error above
• this branch — completes and writes every file (CLAUDE.md, .claude/validate_sandbox.py, notes.md, subdirectories)

yapf --diff clean against .style.yapf; file compiles.

Note for reviewers

PYTHONUTF8=1 is an equivalent workaround for anyone hitting this before the fix lands, but the explicit encoding is the right fix — these files are always UTF-8 regardless of the operator's locale.