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/goal.md

@@ -103,6 +103,13 @@ Goal state changes use round `0`. A continuation consumer attributes each admitt
 /** Message attribution for durable goal state and continuation rounds. */
 interface GoalMessageSource {
   readonly kind: 'goal'
+  /**
+   * Round-zero state changes are `notice`-form contexts; a continuation round
+   * carries the objective forward as ordinary context and declares no form.
+   */
+  readonly form?: 'notice'
+  /** Present with `form`: one-line account of the mutation. */
+  readonly summary?: string
   readonly goalId: GoalId
   readonly revision: number
   /** Zero for state changes; positive for admitted continuation rounds. */