fix(opencode): preserve safe parent permissions in task child sessions

[remorses]

Issue for this PR

Closes #20549

Type of change

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

What does this PR do?

When task creates a child session, it now carries over only the parent session's external_directory rules and deny rules.

That fixes repeated prompts for trusted external paths while avoiding broad allow-rule inheritance that could make restrictive subagents like explore writable.

How did you verify your code works?

• bunx prettier --write packages/opencode/src/tool/task.ts packages/opencode/test/tool/task.test.ts
• bun test --timeout 30000 test/tool/task.test.ts
• bun typecheck

Screenshots / recordings

Not applicable.

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:

Based on my search results, I found related PRs that address similar permission and restriction preservation issues in task child sessions:

Related PRs:

1. fix(task): preserve subagent todo permissions #18202 - fix(task): preserve subagent todo permissions

   • Directly related to preserving permissions when creating subagent sessions

2. fix(opencode): preserve readonly subagent restrictions across compaction #18764 - fix(opencode): preserve readonly subagent restrictions across compaction

   • Addresses preserving restrictions across session operations, similar concern to this PR

3. fix: subagent permissions bypass and Lost restrictions after compaction #21661 - fix: subagent permissions bypass and Lost restrictions after compaction

   • Handles permission and restriction preservation issues

4. fix: avoid external_directory prompt for global AGENTS.md #18721 - fix: avoid external_directory prompt for global AGENTS.md

   • Related to the external_directory prompts that this PR aims to reduce

5. feat: Use proper globbing for "edit", "read" and "external_directory" permi… #22676 - feat: Use proper globbing for "edit", "read" and "external_directory" permissions

   • Recent work on external_directory permission handling

These PRs are related to the same codebase area (task tool, permissions, subagent sessions) but don't appear to be direct duplicates of PR #23290. Your PR is specifically addressing the narrow issue of preserving only safe parent permissions (external_directory and deny rules) when task creates child sessions.