docs: archive historical planning documents and fix references

Move self-described historical/upstream docs to docs/archive/:
- agent-runbook-guide.md
- lan-core-switch-upgrade-plan.md
- lan-rb5009-upgrade.md
- se5420-review-claim-verification-2026-08.md

Update archive/README.md manifest and fix relative links in active docs
and archived docs. Update AGENTS.md docs/ layout description.

docs/lan-overview.md

@@ -47,7 +47,7 @@ from each section below.
 > 交换），全量点只能靠 SE5420 port mirroring。迁移状态见部署计划
 > [lan-se5420-deployment-guide.md](lan-se5420-deployment-guide.md). Design/planning refs:
 > [lan-erx-se5420-network.md](lan-erx-se5420-network.md),
-> [lan-core-switch-upgrade-plan.md](lan-core-switch-upgrade-plan.md).
+> [lan-core-switch-upgrade-plan.md](archive/lan-core-switch-upgrade-plan.md).
 
 ---