feat: secondary sidebar (configurable right panel)

[andrewchumchal]

Summary

Add a secondary sidebar on the right side of the app — a configurable panel region that hosts auxiliary views such as attached context, file references, or conversation outline. Mirrors VS Code's secondary side bar (View > Secondary Side Bar).

Motivation

The primary sidebar holds navigation (Chats, Personas, Marketplace). The main area holds the conversation + optional artifact pane. A right-side panel gives a dedicated surface for supplementary content that benefits from always being visible — attached files, project knowledge, a conversation outline — without cluttering either the sidebar or the chat thread.

Proposed behaviour

Layout position

• Anchored to the far right of the window, next to the main area
• Hidden by default; toggled via activity bar icon or ⌘⇧B
• Resizable via drag handle on its left edge
• Width and open/closed state persisted in uiStore

Hosted views (initial set)

View	Content
Context	Files and knowledge attached to the active conversation
Outline	Headings / sections extracted from the active conversation
Related	Similar past conversations (future: semantic search)

Activity bar integration

• Activity bar gains a secondary icon group (bottom-right aligned, or separate strip) to switch secondary sidebar panels
• Clicking the active icon collapses the secondary sidebar

uiStore changes

• secondarySidebarOpen: boolean
• secondarySidebarWidth: number (persisted to localStorage)
• secondarySidebarPanel: 'context' | 'outline' | 'related'

Acceptance criteria

• Secondary sidebar renders on the right side of the main area
• Toggles with ⌘⇧B and activity bar icon
• Resizable; width persisted across restarts
• Context view shows files/knowledge attached to the active conversation
• Panel selection persisted across restarts
• Collapsing does not shift or break the main area layout

Prerequisites

• feat: app shell — activity bar + primary sidebar panel switching #14 App shell (defines the layout regions)

Related

• Add starter personas and enhance publishing workflow #27 Split main area — together these form the full three-column workspace layout