docs: remove experiments/ and design/ directories

Delete all experiment plans, proposals, research docs, and the
kilo-gateway-integration design doc. These are internal planning
docs that do not belong on the public docs site.

- 12 English experiment files
- 5 zh-CN experiment translations
- 1 design doc (kilo-gateway-integration)
- Remove nav groups from docs.json (English + zh-CN)
- Remove 3 redirects pointing to deleted experiment pages
- Remove dead experiment links from hubs.md

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

docs/docs.json

@@ -535,10 +535,6 @@
       "source": "/onboarding",
       "destination": "/start/onboarding"
     },
-    {
-      "source": "/onboarding-config-protocol",
-      "destination": "/experiments/onboarding-config-protocol"
-    },
     {
       "source": "/pairing",
       "destination": "/channels/pairing"
@@ -559,10 +555,6 @@
       "source": "/presence",
       "destination": "/concepts/presence"
     },
-    {
-      "source": "/proposals/model-config",
-      "destination": "/experiments/proposals/model-config"
-    },
     {
       "source": "/provider-routing",
       "destination": "/channels/channel-routing"
@@ -583,10 +575,6 @@
       "source": "/remote-gateway-readme",
       "destination": "/gateway/remote-gateway-readme"
     },
-    {
-      "source": "/research/memory",
-      "destination": "/experiments/research/memory"
-    },
     {
       "source": "/rpc",
       "destination": "/reference/rpc"
@@ -1358,21 +1346,6 @@
               {
                 "group": "Release policy",
                 "pages": ["reference/RELEASING", "reference/test"]
-              },
-              {
-                "group": "Experiments",
-                "pages": [
-                  "design/kilo-gateway-integration",
-                  "experiments/onboarding-config-protocol",
-                  "experiments/plans/acp-thread-bound-agents",
-                  "experiments/plans/acp-unified-streaming-refactor",
-                  "experiments/plans/browser-evaluate-cdp-refactor",
-                  "experiments/plans/openresponses-gateway",
-                  "experiments/plans/pty-process-supervision",
-                  "experiments/plans/session-binding-channel-agnostic",
-                  "experiments/research/memory",
-                  "experiments/proposals/model-config"
-                ]
               }
             ]
           },
@@ -1938,17 +1911,6 @@
               {
                 "group": "发布策略",
                 "pages": ["zh-CN/reference/RELEASING", "zh-CN/reference/test"]
-              },
-              {
-                "group": "实验性功能",
-                "pages": [
-                  "zh-CN/experiments/onboarding-config-protocol",
-                  "zh-CN/experiments/plans/openresponses-gateway",
-                  "zh-CN/experiments/plans/cron-add-hardening",
-                  "zh-CN/experiments/plans/group-policy-hardening",
-                  "zh-CN/experiments/research/memory",
-                  "zh-CN/experiments/proposals/model-config"
-                ]
               }
             ]
           },