One conversation surface: retire /chat

[LinseCed]

Dependencies

• Blocked by sprintstart-backend#214, which moves filters and named sessions onto the buddy.
• Surfaces the capability mode from sprintstart-backend#206.

Summary & Goal

One conversation surface. A hire asks a question in one place, finds it again in one place, and
turns the buddy's capabilities off when they only want to search.

Technical Specification

• Fold /chat and /buddy into a single page with a conversation list. Both are granted to all
  groups in src/auth/accessPolicy.ts; the retired route redirects rather than 404s, and no
  bookmark breaks.
• The capability switch is the visible half of this. The backend mode exists already;
  surface it as a control on the conversation — capabilities on for a buddy that can act, off for
  plain search. Make the current state obvious, since it changes what the assistant can do.
• Retrieval filters (source system, date range) move from the chat UI onto the unified surface.
• BuddyProvider already holds one conversation for both dock and page
  (src/features/buddy/BuddyProvider.tsx), which is the piece this builds on. The dock's expand
  sequence (aiBuddyBus.ts, announceBuddyPageReady, the growing → covering → revealing handoff)
  must land on the unified page.
• Existing conversations, from both systems, appear in the list. Nothing a hire wrote disappears.
• Retire features/chatbot/* and ChatPage.tsx once nothing references them. Leaving a dead second
  implementation behind reintroduces exactly the confusion being fixed.

Note: searching across chat_messages and buddy_messages is deliberately not part of this.
With one engine there is one place to search, so the problem dissolves rather than being solved.

Acceptance Criteria

• One page serves both uses, with a conversation list
• The retired route redirects; no dead links or 404s
• The capability switch is visible, obvious in its current state, and changes behaviour
• Source and date filters work on the unified surface
• Conversations from both systems appear, with transcripts intact
• The dock's expand lands on the unified page with the handoff animation intact
• The old chat implementation is removed, not left orphaned
• Unit and a11y tests cover the list, the switch, and the filters

Definition of Done

• npm run try passes
• PR reviewed and merged into dev

---

Updated 2026-09-25 (chat → Buddy migration, SprintStartProject/Wiki#319)

This is step 5 of Wiki#319. It was reopened on 2026-09-24 because its criteria were not met when it was closed on Sep 9. Blocked by sprintstart-backend#214 and sprintstart-frontend#266. The text above still stands; these points refine it, verified against dev.

Split with #266. #266 builds the parts: citations (popover and drawer), Stop, queue, reasoning panel, the conversation rail component with search and date grouping, moving the Citation type, and re-pointing the chat callers. This issue wires the page:

• One page, one rail. Conversation history takes BuddyPage's left ConversationRail — the shared rail component already exists and already hosts PM Replies on BuddyPage; this adds the conversation list to it. PM Replies (BuddyPmReplies, which sits there today) move into a ui/SidePanel drawer opened from a header button with an unread badge.
• No project gate. Remove BuddyPage's !selectedProjectId → "No project yet" gate. Without a project, the hire gets an unscoped conversation (Wiki#319 decision 1) instead of a dead end; this matters for everyone redirected from /chat.
• Capability switch lives in BuddyComposer and is sent per message (SendBuddyMessageRequest.capabilitiesEnabled already exists), with its state always visible.
• Filters sit on the composer and apply to the active conversation.
• Dock. BuddyProvider holds one conversation; with several, the dock shows the most recently active one. "New conversation" in the dock creates one, and expanding navigates to /buddy/:id of that conversation, with the growing → covering → revealing handoff intact.
• Redirects. /chat → /buddy and /chat/:id → /buddy/:id (chat UUIDs are kept by the backfill in Show the closed-source state on the current-task card #259). Update accessPolicy and AppRouter.
• Delete features/chatbot, ChatPage, ChatProvider, chatService and their tests once Buddy parity with chat: citations, stop, queue, reasoning, searchable rail, re-point chat consumers #266 has moved the callers.
• Not ported (decided): KaTeX, aria-live turn announcements, chat mascot animations.
• Order: land sprintstart-frontend#267 first (the tutor PR that replaced closed Feature/311 buddy onboarding tutor #265; approved — merge dev in, BuddyComposer/useBuddyConversation conflict), then fix(buddy): edit the flag-to-PM question before confirming (#235) #272 (stacked on it). Easter Eggs Consolidation & Overhaul #184 and Prevent Buddy composer lag in long conversations #236 are already merged.

Added criteria:

• PM Replies open from the header drawer, with an unread badge
• A hire without a project can open and use an unscoped conversation
• The dock shows the most recent conversation and expands to its /buddy/:id