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/agent_platform_port.py

@@ -52,6 +52,7 @@ from web.paperclip_client import (
     post_comment as pc_post_comment,
     reject_interaction as pc_reject_interaction,
     reset_agent_session as pc_reset_agent_session,
+    reset_case_agents as pc_reset_case_agents,
     respond_to_interaction as pc_respond_to_interaction,
     restore_project as pc_restore_project,
     update_project_name as pc_update_project_name,
@@ -114,4 +115,5 @@ __all__ = [
     "pc_get_run_events",
     "pc_cancel_run",
     "pc_reset_agent_session",
+    "pc_reset_case_agents",
 ]