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/tasks/tool-tasks/src/index.ts

@@ -114,6 +114,26 @@ function fitWithSuffix(
   return `${retainTail(content, maxBytes - fixedBytes)}${fixed}`
 }
 
+/**
+ * Bound for the durable one-line account. A notice summary rides a collapsed
+ * transcript row, and both the task label and its status detail are caller
+ * text with no length of their own, so the summary caps itself rather than
+ * committing unbounded prose to the log.
+ */
+const SUMMARY_MAX_CHARS = 120
+
+/**
+ * One-line account of a settled task for the `notice` form's collapsed row.
+ * @param snapshot - the settled task.
+ * @returns its kind, label, and status, bounded to {@link SUMMARY_MAX_CHARS}.
+ */
+function completionSummary(snapshot: TaskSnapshot): string {
+  const summary = `${snapshot.kind} ${snapshot.label} ${statusLine(snapshot)}`
+  return summary.length <= SUMMARY_MAX_CHARS
+    ? summary
+    : `${summary.slice(0, SUMMARY_MAX_CHARS - 1)}…`
+}
+
 function fitCompletionNotice(snapshot: TaskSnapshot): string {
   const prefix = `background task ${snapshot.id}`
   const detail = ` (${snapshot.kind}: ${snapshot.label}) finished ${statusLine(snapshot)}`
@@ -230,7 +250,12 @@ export function apply(ctx: Context, config: Config): void {
         type: 'text',
         text: fitCompletionNotice(snapshot),
       }],
-      source: { kind: 'plugin', plugin: 'tool-tasks' },
+      source: {
+        kind: 'plugin',
+        plugin: 'tool-tasks',
+        form: 'notice',
+        summary: completionSummary(snapshot),
+      },
     }))
   })