Add sidebar indicators for sessions needing input and newly completed ACP turns

[turingcat]

Problem

The conversation sidebar only shows a spinner while an ACP turn is running. This leaves two important states hard to discover:

1. A session can be blocked on a permission request, question, structured question, or plan approval while still looking like a generic running session.
2. Once a background turn completes, the spinner disappears and the session becomes visually indistinguishable from older history, so users cannot tell which work just finished.

Expected behavior

• Show a persistent amber bell and subtle amber row highlight when an open ACP session needs user interaction.
• Show a persistent green completion icon and subtle green row highlight when a background ACP turn transitions from prompting to connected.
• Treat a turn completed in the currently active conversation as already read.
• Clear the completion indicator when the user opens that conversation.
• Clear stale completion state when a new prompt starts.
• Give interaction-required state higher priority than completion state.
• Track only currently open ACP conversations and do not backfill historical completed sessions on startup.

This follows the activity model used by multi-session terminal sidebars: running, waiting for input, and newly completed work should remain visually distinct.