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.

docs/core-data-structures/subagent.md

@@ -149,6 +149,8 @@ Final settlement awaits `ctx.sessions.flush(session)` but ignores its participat
 /** Attribution for a model coordinator's follow-up to one of its children. */
 interface CoordinatorMessageSource {
   readonly kind: 'coordinator'
+  /** A message another agent addressed to this one (`relay` context form). */
+  readonly form: 'relay'
   /** Session id of the agent whose tool call produced the follow-up. */
   readonly senderSessionId: SessionId
 }
@@ -182,6 +184,8 @@ An optional continuable-child setup contribution can install scope-local capabil
 /** Durable attribution for a continuable child's explicit parent report. */
 interface SubagentReportMessageSource {
   readonly kind: 'subagent-report'
+  /** A message another agent addressed to this one (`relay` context form). */
+  readonly form: 'relay'
   /** Session id of the reporting child. */
   readonly senderSessionId: SessionId
 }