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/context/time-context/src/index.ts

@@ -174,6 +174,10 @@ export function apply(ctx: Context, config: Config): void {
     const previous = step === 1
       ? precedingMessageTime(agent)
       : precedingStepContextTime(agent, turn)
-    agent.inject(createUserMessage({ content: [{ type: 'text', text: renderText(now, turn, step, previous, formatter, resolvedTimeZone) }], source: { kind: 'plugin', plugin: name } }))
+    const text = renderText(now, turn, step, previous, formatter, resolvedTimeZone)
+    agent.inject(createUserMessage({
+      content: [{ type: 'text', text }],
+      source: { kind: 'plugin', plugin: name, form: 'snapshot', sections: [{ name, text }] },
+    }))
   }, { prepend: true })
 }

packages/context/time-context/tests/time-context.spec.ts

@@ -154,7 +154,19 @@ describe('durable step context', () => {
     const event = session.events.at(-1)
     expect(event?.type).toBe('user/message')
     if (event?.type !== 'user/message') throw new Error('missing time context')
-    expect(event.data.source).toEqual({ kind: 'plugin', plugin: 'time-context' })
+    // The reading is a `snapshot`-form context: one named contribution whose
+    // text is exactly what the model read, so a consumer attributes it without
+    // re-splitting prose.
+    expect(event.data.source).toEqual({
+      kind: 'plugin',
+      plugin: 'time-context',
+      form: 'snapshot',
+      sections: [{
+        name: 'time-context',
+        text: 'Time sampled while preparing turn 1, step 1: 2026-07-15T09:01:01+08:00[Asia/Shanghai]\n'
+          + 'Elapsed since the preceding model-visible message: 1d 1h 1m 1s.',
+      }],
+    })
     expect(event.surfaceOp).toBe('append')
   })