feat(web): declare the remaining context forms on every shipped producer

Four values complete the vocabulary, so the opaque body is reached only by
producers that genuinely promise no shape.

`snapshot` — current state a later snapshot supersedes. system-prompt now
exposes `renderContextSections()`, the named contributions
`renderContextSnapshot()` already joins for the model, so the body attributes
each part to the subsystem that produced it instead of re-splitting joined
prose. The runtime snapshot, time-context, and tmux-context declare it.

`notice` — a one-off account of what just happened, declared by tool-tasks,
goal state changes, tool-goal wrap-up, plan-mode switches, and
repeat-tool-guard. Its `summary` rides the COLLAPSED row: these five are the
majority of shipped producers and none of them needs expanding to be read.
The task summary bounds itself because its inputs are unbounded caller text.

`relay` — a message another agent addressed to this one; both subagent
sources declare it and the body names the sender above what it said.

`recall` — material lifted from another session's log. session-reference
needed no new field: its references already record retained and omitted
counts and the truncation flag, which the body shows first, because recalled
context is bounded on the way in.

`ContextFormed` is now discriminated by `form`, so a producer cannot declare
a shape without the facts that shape is presented from — a notice without its
summary, or a snapshot without its sections, fails to compile.

Only the two hook bridges stay opaque, by design: their content is whatever
an external program printed, so no shape can be promised for it. Unknown
kinds and unreadable records land there too.

packages/plan/plan-mode/src/index.ts

@@ -454,7 +454,8 @@ export class PlanModeService extends Service {
       : 'The user switched this session back to the default mode.'
     session.append('user/message', createUserMessage({
       content: [{ type: 'text', text }],
-      source: { kind: 'plugin', plugin: 'plan-mode' },
+      // The narration is already one sentence, so it is its own summary.
+      source: { kind: 'plugin', plugin: 'plan-mode', form: 'notice', summary: text },
     }), { surfaceOp: 'append' })
   }
 }