bug: Workspace Main after Reset shows only the reset notice, not the new-chat empty state #3795

Problem

After Reset on an agent's Main chat in the Workspace, the fresh Main shows only the centred system line

Main was reset. The previous conversation is saved as "…".

and nothing else — the new-chat empty state (briefing, hints, suggestions) never appears, so the reset chat looks blank instead of inviting a first message.

Root cause

reset_main_session (src/backend/client_portal/service.py) writes one role: "system" message (MAIN_RESET_NOTICE) into the freshly minted Main. The conversation renders its empty slot only when messages.length === 0 (src/frontend/src/components/portal/PortalConversation.vue), so that one signpost suppresses the whole new-chat state.

Everything else already treats the row as empty: the notice does not bump message_count/last_message_at, so the landing rule (agentEmptyChat) and Reset's "already fresh" no-op both see a fresh chat. Only the renderer disagrees.

Acceptance Criteria

• A chat whose only messages are role: "system" lines renders the new-chat empty state (briefing + suggestions)
• The reset system line stays visible (muted signpost) alongside the empty state
• A chat with any user/assistant message renders no empty state (unchanged)
• Unit test pins the "empty = no non-system messages" rule