fix(security): prevent path traversal via symlinks

[google-labs-jules]

Summary

This PR fixes a critical security vulnerability that allowed directory traversal via symbolic links. The File.read and File.list tools previously relied on lexical path validation (path.relative), which could be bypassed by creating a symlink inside the project pointing to an external file or directory.

What was broken

• File.read allowed reading files outside the project if accessed via a symlink.
• File.list allowed listing directories outside the project if accessed via a symlink.
• Instance.containsPath (lexical check) returned true for these symlinks because it did not resolve them.

Fix approach

• In packages/opencode/src/file/index.ts:
  • Update File.read to resolve the path using fs.promises.realpath after confirming existence but before reading content.
  • Update File.list to resolve the path using fs.promises.realpath before listing content.
  • Verify that the resolved real path is contained within the project directory using Instance.containsPath.

Verification

• Added a new test case packages/opencode/test/security/symlink.test.ts that:
  1. Creates a secret file outside the project.
  2. Creates a symlink to it inside the project.
  3. Attempts to read the symlink using File.read.
  4. Asserts that the operation fails with "Access denied".
• Ran the test and confirmed it passes (access is denied).
• Verified that valid operations still work.

Checklist

• Fix implemented in File.read and File.list
• Regression test added
• Verified fix with test

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PR created automatically by Jules for task 9454945919874623948 started by @Ashwinhegde19

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