feat(web): Show subagents under parent session, allow intuitive navigation between.

[jmagder]

What does this PR do?

Closes #14040

This PR improves the opencode web client by adding support for navigation between sessions and their subgents, through an intuitive UX:

You can watch a video demo here:

2026-02-17.17-31-40.mp4

In short:

• subagents are now shown as a tree under their parent session.
• The session can be collapsed.
• You can navigate to parent session or subagent through the sidebar navigation
• Subagent threads now have an icon in top right to show they are subagents
• Subagent threads can now navigate directly to their parent.

How did you verify your code works?

Aside from extensive testing, I added unit and e2e test for the feature. I also put together the video above demoing the feature.

The actual code changes are a few hundred lines. The tests take up 70% of the code, but if you are short on time less focus is needed on the *.spec.ts files.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

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

See CONTRIBUTING.md for details.

[github-actions]

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

Based on my search, I found several related PRs that address subagent navigation features:

Potentially Related PRs:

1. feat: add subagents sidebar with clickable navigation and parent keybind #4865 - "feat: add subagents sidebar with clickable navigation and parent keybind"

   • feat: add subagents sidebar with clickable navigation and parent keybind #4865
   • Directly related: Implements sidebar navigation for subagents with parent navigation capability

2. feat: add interactive session switcher for subagent navigation #6184 - "feat: add interactive session switcher for subagent navigation"

   • feat: add interactive session switcher for subagent navigation #6184
   • Related: Provides session switching functionality for subagent navigation

3. feat(task): Add subagent-to-subagent delegation with budgets, persistent sessions, and hierarchical session navigation #7756 - "feat(task): Add subagent-to-subagent delegation with budgets, persistent sessions, and hierarchical session navigation"

   • feat(task): Add subagent-to-subagent delegation with budgets, persistent sessions, and hierarchical session navigation #7756
   • Related: Includes hierarchical session navigation for subagents

4. Add session position indicator in header and refactor sibling logic #5309 - "Add session position indicator in header and refactor sibling logic"

   • Add session position indicator in header and refactor sibling logic #5309
   • Potentially related: Session positioning and navigation logic

These PRs appear to address similar or overlapping features around subagent navigation and hierarchical session display. You may want to review them to ensure #14043 doesn't duplicate existing functionality.