feat: add sidekick - parallel chat companion for active sessions #20713

Is your feature request related to a problem?

When working with opencode on long-running agent tasks, users have no way to discuss or review the agent's progress in real-time. They must wait for the task to complete before they can comment, ask questions, or course-correct. This creates a frustrating idle period during complex operations.

Describe the solution you'd like

A Sidekick feature: a lightweight, parallel chat-only conversation that runs alongside the main session. The sidekick:

• Reads the parent session's messages in real-time (snapshot at prompt time)
• Has its own independent context window (no shared context pollution)
• Cannot execute any tools (chat-only companion)
• Allows users to inject sidekick conclusions back into the parent session's context
• Appears as a sidebar tab in the TUI (no layout changes needed)

This gives users a "waiting room" experience where they can monitor, discuss, or simply chat while the main agent works.

Describe alternatives you've considered

1. Separate terminal session: Users could open a second opencode instance, but it wouldn't have access to the main session's context.
2. In-session comments: Adding comments to the main session would pollute the agent's context window and potentially confuse the model.
3. Read-only viewer: A viewer without chat capability, but this doesn't allow users to discuss or annotate progress.

Additional context

Implementation is ready as a PR. The sidekick is modeled as a child session with kind: "sidekick" and parentID pointing to the main session, with comprehensive route guards, tool blocking, and TUI integration.