Add pinned session pills above the chat input
### Summary
Add a row of pinned pills above the input area in Duo Chat, showing **currently active** sessions only. Pills are removed when their session completes. Inline session cards in the message thread already handle the persistent record of past sessions; this bar is for live context (Running, Pending etc).
This issue also relocates the "Responses may be inaccurate. Verify before use." disclaimer to make room for the pill bar.
### Behavior
[Figma](https://www.figma.com/design/VqpH0V6joIOokfeXS71Yqz/Duo-Agent-Platform-Side-Panel?m=auto&node-id=18668-73449&t=J6t8Y0uqN7OABXzd-1)
{width=900 height=434}
#### Active session pills
* Horizontal row of pills sits directly above the input area.
* Only **active sessions** appear here. When a session completes (finished, or otherwise terminal), its pill drops from the bar.
* Each pill: small status dot (color-coded by state) + truncated session title.
* Pills are clickable; clicking opens that session in the side panel.
* Overflow handled with a trailing `•••` button when pill count exceeds available width.
* The pill bar reserves vertical space so its presence or absence does not shift the layout when sessions come and go.
#### Disclaimer relocation
* "Responses may be inaccurate. Verify before use." currently sits in the area now occupied by the pill bar.
* Move the disclaimer to the bottom of the input area (below the input field), or hide it entirely pending legal review on whether it's strictly required in that position.
### Design rationale
* **Active-only keeps the bar honest as a "right now" surface.** Stale finished pills would dilute the signal that this is your live working context.
* **Completed sessions still have a home in the thread** as inline cards, so dropping them from the pill bar isn't a loss of access — it's a separation of concerns between "live context near input" and "historical record in thread."
* **Surfaces active session context near the input,** where other current-context controls already live (model selector, attachments).
* **Lets users switch between active sessions without scrolling back through the thread** to locate the originating inline card.
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