feat(client-runtime): project the archive set and the archiveSession action

WorkspaceListState gains archivedSessionIds (ReadonlySet, replaced only
on membership change), installed as full snapshots from the list
baseline, the unary echo, and the changed frame. Archiving the current
session clears the selection into the New Session view state. Test
doubles (test-runtime, fake APIs, fixture client) follow the widened
IWorkspaces/IApiClient faces.

packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx

@@ -74,7 +74,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(
@@ -111,7 +111,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(