fix: use relative paths for external_directory permission patterns

Summary

This PR fixes the external_directory permission to use relative paths instead of absolute paths, making it consistent with other permissions like edit and allowing users to configure patterns like ../sibling-project-*.

Problem

The external_directory permission was using absolute paths internally, which meant user-configured relative path patterns would never match. For example:

{
  "permission": {
    "external_directory": {
      "*": "ask",
      "../sibling-project-*": "allow"
    }
  }
}

This config would not work because when accessing /Users/foo/sibling-project-bar/file.txt from /Users/foo/myproject, the internal pattern would be /Users/foo/sibling-project-bar/*, which doesn't match ../sibling-project-*.

Solution

Changed assertExternalDirectory to compute relative paths using path.relative(Instance.directory, parentDir):

// Before
const parentDir = kind === "directory" ? target : path.dirname(target)
const glob = path.join(parentDir, "*")

// After  
const parentDir = kind === "directory" ? target : path.dirname(target)
const relativeParentDir = path.relative(Instance.directory, parentDir)
const glob = path.join(relativeParentDir, "*")

Now when accessing /Users/foo/sibling-project-bar/file.txt from /Users/foo/myproject, the pattern becomes ../sibling-project-bar/*, which correctly matches ../sibling-project-*.

Rationale

1. Consistency: The edit permission already uses relative paths (path.relative(Instance.worktree, filePath)), so external_directory should behave the same way

2. Principle of least surprise: The documentation shows relative path examples, and users reasonably expect them to work

3. Portability: Relative paths work across machines, while absolute paths are machine-specific

4. Minimal change: The fix is a single-line addition that converts the path before building the glob pattern

Changes

• src/tool/external-directory.ts: Use path.relative() to compute relative path patterns
• test/tool/external-directory.test.ts: Updated test expectations for relative paths
• test/tool/read.test.ts: Updated test to check for relative path format

Testing

All existing tests pass with updated expectations:

• test/tool/external-directory.test.ts - 5 tests
• test/tool/read.test.ts - 26 tests
• test/tool/bash.test.ts - 12 tests (unchanged)
• test/agent/agent.test.ts - 34 tests

Note

The bash tool has separate external_directory logic that still uses absolute paths. This can be addressed in a follow-up PR to keep this change tightly scoped.