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Sweep the silently auto-merged leftovers of the superseded #587 wire layer: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.

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -45,8 +45,6 @@ function scriptedApi(overrides: {
       }),
       prompt: r => ok(r, { accepted: true as const }),
       cancel: r => ok(r, { accepted: true as const }),
-      planMode: r => ok(r, null),
-      setPlanMode: r => ok(r, null),
       ...overrides.sessions,
     },
     host: {