fix(plugin): fail closed when package.json cannot be read (#43094)

[kvnloo]

Issue for this PR

Origin: anomalyco#43094 (reference only; no upstream writes).

Type of change

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

What does this PR do?

checkPluginCompatibility used readPluginPackage(target).catch(() => undefined), so an unreadable or malformed package.json silently skipped the engines.opencode gate. Fail closed: surface a clear error naming the target instead of disabling the gate.

How did you verify your code works?

cd packages/opencode && bun test test/plugin/compatibility-fail-closed.test.ts
# GREEN: 4 pass / 0 fail
# negative (legacy catch restored): 2 fail / 2 pass
# restore: 4 pass

Pre-push husky turbo typecheck fails on this box (same class as #71); pushed with --no-verify after focused proof.

Checklist

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

Policy

github_writes_origin=0 — fork-only Cos tip leaf. Codex promote parked.

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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