fix: correctly read cache metadata as UTF-8

[Pililink]

Summary

• read cached metadata.json with explicit UTF-8 encoding during cache validation
• add a regression test for non-ASCII cached file paths under a simulated CP936 default encoding

Root cause

SembleIndex.save() writes cache metadata as UTF-8 JSON bytes, but get_validated_cache() read the same file using the platform default text encoding. On Windows systems using CP936/GBK, UTF-8 metadata containing non-ASCII file paths could fail during cache validation before the cached index was loaded.

Fixes #202

Validation

• uv run --extra dev pytest tests/test_cache.py
• uv run --extra dev ruff check src/semble/cache.py tests/test_cache.py
• uv run --extra dev --extra mcp pytest -k "not test_walk_files_skips_symlinks"
• Manual CLI repro: two consecutive searches against a temp repo containing docs\\测试检查清单.md with PYTHONUTF8 and PYTHONIOENCODING unset

Note: full uv run --extra dev --extra mcp pytest reaches 271 passed and then fails only on tests/test_file_walker.py::test_walk_files_skips_symlinks because this Windows environment lacks permission to create symlinks (WinError 1314).

[greptile-apps]

Confidence Score: 5/5

Safe to merge — one-line change with a targeted regression test that correctly exercises the exact failure mode.

The change is minimal: a single encoding="utf-8" argument added to one open() call. The rest of the codebase already uses binary mode for persistence I/O, so there are no other text-encoding gaps to worry about. The regression test is well-constructed — it writes UTF-8 metadata, patches builtins.open to inject CP936 only when no encoding is supplied, and uses a git URL to skip file-walking, exercising exactly the path that was broken.

No files require special attention.

_{Reviews (1): Last reviewed commit: "修复 Windows 缓存元数据编码读取" | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/cache.py	100.00% <100.00%> (ø)

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