[Feature] Restore in-sidebar channel list (or make it optional vs the popover)

Summary

The chat/conversation switcher in the agent portal is now a popover (PortalHistoryPopover) rather than a persistent list. For users with many channels per agent, this loses at-a-glance visibility into unread state, recently-active channels, and which conversation is currently selected. A persistent list (or at least an option to enable one) would close that gap.

Current state

• interface/src/routes/AgentChat.tsx is a 5-line shim that renders <PortalPanel agentId={…} />
• Channel/conversation switching lives in interface/src/components/portal/PortalHeader.tsx:116-130 as a PopoverRoot wrapping PortalHistoryPopover — click-to-open dropdown
• The sidebar (interface/src/components/Sidebar.tsx:67-74) only exposes top-level agent sub-nav: Chat | Channels | Memory | Skills | Schedule | Config — no per-channel entries
• This was introduced in commit c53a1e38 (2026-04-04, "SpaceUI migration: sidebar redesign…") which redesigned the sidebar to a fixed-shape accordion. Prior to that, channels were visible directly in the sidebar.

Proposed change

User-configurable switcher style:

• New setting (Settings → Appearance, or per-agent): Channel switcher: sidebar | popover (default: keep current popover to avoid surprise; or flip if maintainers prefer)
• When sidebar, render channel list under the agent in Sidebar.tsx with last-message preview + unread badge, similar to the pre-c53a1e38 layout
• When popover, keep today's PortalHistoryPopover exactly as-is
• Persist setting in localStorage (or per-user once #584 lands)

Related

• #458 — proposes a different sidebar concept ("Migrate sidebar from Agents to Humans"). Worth coordinating: a configurable switcher style fits inside either layout.
• #439 — thread support in agent chat. Threads inside a channel would benefit even more from a sidebar-style hierarchical view.

Open questions

• Was the popover an intentional simplification, or a side-effect of the SpaceUI migration? If the latter, should the default be sidebar for power users with >5 channels per agent?

Start with interface/src/components/portal/PortalHeader.tsx, interface/src/components/Sidebar.tsx, and PortalHistoryPopover to map the current switcher and sidebar behavior. Review the pre-c53a1e38 layout and coordinate with #458 and #439 before choosing the setting scope, default, and persistence approach. Done means the selected switcher style works, preserves the current popover option, and shows the requested channel information in sidebar mode.