feat(agents): reset button + endpoint to clear stuck agent error state

Adds a one-click 'reset agents' action for cases where writer/QA agents
are stuck in Paperclip's error state (triggered by recovery loop or
failed run). Addresses the root cause documented in
reference_paperclip_recovery_loops: reassigns open issues from agents
back to the chair user, and calls reset_agent_session for each error
agent to clear wedged runtime sessions.

Changes:
- paperclip_client.py: reset_case_agents() — reassigns stuck issues +
  clears error status in Paperclip DB + calls reset_agent_session
- agent_platform_port.py: exports pc_reset_case_agents (G12 gate)
- app.py: POST /api/cases/{case_number}/agents/reset endpoint
- agents.ts: useResetCaseAgents mutation hook + AgentResetResult type
- agent-status-widget.tsx: 'אפס' button (shown only when error agents
  exist) with Dialog confirmation + loading state + toast feedback

Invariants: G12 (Paperclip only via port), G2 (no parallel path —
uses existing reset_agent_session + pc_get_case_issues).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web-ui/src/lib/api/agents.ts

@@ -179,3 +179,25 @@ export function useSubmitInteraction(caseNumber: string | undefined) {
     },
   });
 }
+
+export type AgentResetResult = {
+  ok: boolean;
+  reassigned_issues: { id: string; identifier: string }[];
+  reset_agents: { id: string; name: string; ok: boolean; error?: string }[];
+};
+
+export function useResetCaseAgents(caseNumber: string | undefined) {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: () =>
+      apiRequest<AgentResetResult>(
+        `/api/cases/${caseNumber}/agents/reset`,
+        { method: "POST" },
+      ),
+    onSuccess: () => {
+      if (caseNumber) {
+        qc.invalidateQueries({ queryKey: agentKeys.activity(caseNumber) });
+      }
+    },
+  });
+}