fix(core): decode UTF-16 BOMs in readJson

[reynold-hu]

Issue for this PR

Closes #54306

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

FSUtil.readJson decoded every file as UTF-8. JSON written as UTF-16 with a BOM — the default output of Windows shell redirection, e.g. PowerShell 5.1 opencode export <id> > session.json — turned into replacement characters and failed with Invalid JSON ... Unrecognized token.

It now decodes BOM-prefixed UTF-16LE/UTF-16BE files and otherwise keeps the previous behavior (UTF-8, with a UTF-8 BOM stripped). UTF-16 without a BOM stays unsupported because the encoding is ambiguous.

How did you verify your code works?

• Added tests in packages/core/test/filesystem/filesystem.test.ts covering UTF-16LE, UTF-16BE and UTF-8 BOM files; they fail before the change and pass after.
• End-to-end on Windows 10: bun dev export <id> > session.json then bun dev import session.json prints Imported session: ...; before the change it failed with Invalid JSON ... Unrecognized token.
• bun typecheck in packages/core passes.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[opencode-agent]

Thanks for the clear write-up and the tests, @reynold-hu!

v1 (the dev branch) only takes critical fixes. This bug has a simple workaround: write or convert the export as UTF-8 (for example opencode export <id> | Out-File -Encoding utf8 session.json, or use PowerShell 7), so it doesn't qualify.

The same issue seems to exist in v2: readJson in packages/util/src/fs-util.ts on the v2 branch also reads files with readFileString. You're welcome to open this fix against v2. It's a separate implementation, so the change would need to be adapted there.