Adds documentation for the Terminal Threads feature — the ability to
open interactive terminals in the Agent Panel alongside threads.

## Changes

- **`agent-panel.md`**: Adds a new `## Terminals` section covering how
to open terminals, terminal titles, bell notifications, and closing.
Also adds a Terminal bullet to the `+` menu description in "Creating New
Threads".
- **`parallel-agents.md`**: Updates the page title, description, and
opening paragraph to include terminals alongside threads. Adds a note in
the Threads Sidebar section that terminals appear there too.

Release Notes:

- N/A

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Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>