feat(web): Phase 1-UX frontend — DiffPanel agent badges + resumed/new-session chip

DiffPanel renders a per-row agent badge (icon+label; null -> 'manual') + a 'Changes from X, Y' note when the pending set spans >1 agent. AgentComposerBar gains an optional sessionId prop -> resumed/history/new-session chip beside the Provider picker (gated, so BooChat callers are unchanged), driven by a new useAgentSessions hook (refetch on message-complete). providerIcon extracted to shared components/coder/providerIcons.tsx; api.coder gains agentSessions(sessionId); PendingChange type gains agent. web tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/api/client.ts

@@ -25,6 +25,17 @@ import type {
   WorkspaceState,
 } from './types';
 
+// v2.6 Phase 1-UX §9b: chat-scoped agent-session rows. Returned by
+// GET /api/coder/sessions/:id/agent-sessions; drives the AgentComposerBar
+// resumed/new-session chip via useAgentSessions. `has_session` is true when a
+// resumable backend session id exists for that agent in the chat.
+export interface AgentSessionInfo {
+  agent: string;
+  status: string;
+  has_session: boolean;
+  last_active_at: string | null;
+}
+
 export class ApiError extends Error {
   constructor(
     public status: number,
@@ -363,6 +374,11 @@ export const api = {
       request<CoderMessageWire[]>(
         `/api/coder/sessions/${sessionId}/messages${chatId ? `?chat_id=${encodeURIComponent(chatId)}` : ''}`,
       ),
+    // v2.6 Phase 1-UX §9b: per-(chat,agent) backend-session state for the
+    // resumed/new-session chip. Chat-scoped (NOT foldable into the project-level
+    // provider snapshot). Proxied to boocoder at /api/sessions/:id/agent-sessions.
+    agentSessions: (sessionId: string) =>
+      request<AgentSessionInfo[]>(`/api/coder/sessions/${sessionId}/agent-sessions`),
     skillInvoke: (
       sessionId: string,
       paneId: string,