List Trashed Agents

List the caller's recycle-binned agents (TOS-397).

Declared before GET /{agent_id} so the literal trash segment isn't captured as an agent id.

TOS-3512 follow-up: trimmed to AgentSummaryResponse like the other two list endpoints. AgentsPage.tsx renders trashed agents through the same AgentCard as the live list, reading knowledge_source_count rather than the full config this shape omits.

TOS-3663 follow-up: routed through _agent_list_payloads like every other list. list_deleted_agents_for is scoped to agents the caller administers, so in practice every row here is an owner row and nothing is stripped - but that is a property of today's query, not of this endpoint, and pinning the serializer is what keeps it true if the scope ever widens. The move also removes a real N+1: this endpoint passed no tags, so _agent_to_response fired one agent_tags query per trashed agent.