chore(web): drop plan RPC residue and stale carriers after merge

Sweep the silently auto-merged leftovers of the superseded #587 wire layer:
the PlanModeState interface and planMode/setPlanMode entries in the sessions
API and rpc map, the fixture plan RPC spec, the composer.controls README
paragraph and spec assertions, the old-format plan-mode snapshot, and the
two Agent Notes written against the RPC design (to be rewritten for the
projection reattachment). ChatView/PendingCard keep the approval-only
pending rendering; QuestionComposer improvements ride along unchanged.

apps/web/tests/snapshots/plan-mode.json

@@ -1,44 +0,0 @@
-{
-  "initial": {
-    "stage": "initial",
-    "label": "默认",
-    "value": "default",
-    "title": "当前为默认模式",
-    "primary": "发送"
-  },
-  "planPending": {
-    "stage": "plan-pending",
-    "label": "计划 · 待生效",
-    "value": "plan",
-    "title": "当前为默认模式；计划模式将在下一次模型请求时生效",
-    "primary": "发送"
-  },
-  "planCommitted": {
-    "stage": "plan-committed",
-    "label": "计划",
-    "value": "plan",
-    "title": "当前为计划模式",
-    "primary": "停止"
-  },
-  "defaultPendingWhileRunning": {
-    "stage": "default-pending-running",
-    "label": "默认 · 待生效",
-    "value": "default",
-    "title": "当前为计划模式；默认模式将在下一次模型请求时生效",
-    "primary": "停止"
-  },
-  "defaultPendingAfterStop": {
-    "stage": "default-pending-stopped",
-    "label": "默认 · 待生效",
-    "value": "default",
-    "title": "当前为计划模式；默认模式将在下一次模型请求时生效",
-    "primary": "发送"
-  },
-  "defaultCommitted": {
-    "stage": "default-committed",
-    "label": "默认",
-    "value": "default",
-    "title": "当前为默认模式",
-    "primary": "停止"
-  }
-}