fix(shell_hooks): use explicit UTF-8 encoding with errors='replace'

[seasonmsg]

Problem

On Windows, text=True in subprocess.run() defaults to the system's ANSI codepage (e.g., GBK/CP936 on Chinese Windows). When shell hook scripts output characters outside this codepage, subprocess.run raises UnicodeDecodeError, causing the hook to silently fail.

Fix

Replace text=True with explicit encoding='utf-8', errors='replace' in _spawn() (shell_hooks.py:467).

• encoding='utf-8': consistent cross-platform behavior
• errors='replace': graceful degradation instead of crash on invalid bytes

Before/After

# Before
subprocess.run(argv, input=stdin_json, capture_output=True,
               timeout=spec.timeout, text=True, shell=False, **_popen_kwargs)

# After
subprocess.run(argv, input=stdin_json, capture_output=True,
               timeout=spec.timeout, encoding='utf-8', errors='replace',
               shell=False, **_popen_kwargs)

Testing

• Verified that _spawn() no longer crashes on non-ASCII hook output on Windows (Chinese locale)
• Backward compatible: encoding='utf-8' + errors='replace' is a superset of text=True behavior on UTF-8 systems

[teknium1]

Thanks for targeting the shared shell-hook subprocess bridge. The reported call remains on current main at agent/shell_hooks.py:462-470, and this is a small, focused fix.

Problems

• The PR adds no regression test for the new encoding contract. The existing Windows hook test at tests/test_windows_subprocess_no_window_flags.py:264-283 verifies only creationflags.
• The public hook protocol documents JSON stdin/stdout at website/docs/user-guide/features/hooks.md:1325-1361, but does not state an encoding. Replacing locale decoding with UTF-8 changes behavior for hooks that emit locale-encoded JSON, so the backward-compatibility claim needs an explicit protocol decision.

Suggested changes

• Add a focused captured-kwargs test asserting encoding='utf-8', errors='replace', and no text=True.
• Document UTF-8 as the hook wire encoding, or preserve locale-encoded-hook compatibility if that remains supported.

Automated hermes-sweeper review.

[teknium1]

Closing as resolved by PR #70875 (merged, commit 0f732cb), which closed out this bug class codebase-wide: every text=True subprocess call now passes encoding="utf-8", errors="replace", and a CI linter rule (scripts/check-windows-footguns.py) rejects any future unguarded site. The sites this PR targeted are all guarded on current main — verified per-file. Credit for the class fix goes to @Stoltemberg (#55339, the original sweep) and @jinglun010-cpu (#60741 + the #60751 linter); thanks for your fix as well — the volume of independent PRs on this bug is what escalated it to a class-wide close-out.