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/session-reference/src/index.ts

@@ -199,6 +199,7 @@ export class SessionReferenceService extends Service {
     const prompt = renderPrompt(rendered.map(source => source.data))
     const source: SessionReferenceSource = {
       kind: 'session-reference',
+      form: 'recall',
       version: 1,
       references: rendered.map((source, index) => ({
         sessionId: source.data.sessionId,

packages/context/session-reference/src/types.ts

@@ -6,6 +6,8 @@ import type { SessionId, UserMessage } from '@deepseek-ai/dsh-session'
 /** Durable provenance for one prepared cross-session context. */
 export interface SessionReferenceSource {
   kind: 'session-reference'
+  /** Material lifted out of another session's log (`recall` context form). */
+  form: 'recall'
   version: 1
   references: {
     sessionId: string