fix(web): address the review round on the remaining context forms

- `relay` resolves its sender in `contextBody` like every other form. It was
  the one shape whose marker could claim a form the body did not render: an
  unreadable sender fell back inside the body while the row still said relay,
  contradicting the contract this PR's own note states.
- `recall` requires the retained, omitted, and truncated fields. Completeness
  is what the card exists to report, so a reference that cannot state it is
  not a readable recall — showing the label alone presents a confident card
  over unknown loss.
- The snapshot body states the supersession its producer framing line carries.
  That line is the one part of the model-facing text no section contains, and
  unlike an instruction context's `<system-reminder>` it states the form's own
  semantics rather than wrapping content.
- `GoalMessageSource` is a discriminated pair, so `{ form: 'notice' }` without
  its account no longer compiles. The guarantee this PR claims now holds at
  that seam too, not only through `ContextFormed` on plugin sources.
- Goal and tool-goal summaries are bounded by a shared `boundContextSummary`,
  which tool-tasks now uses as well. A goal objective is unbounded caller text
  in exactly the way a task label is.
- The runtime snapshot interpolates once per request: agent-loop renders the
  sections and joins them through `joinContextSections`.
- Every form's fallback branch is pinned, not only the notice one.

packages/llm/llm/src/message.ts

@@ -104,6 +104,24 @@ export interface MessageSourceMap {
   tool: ToolMessageSource
 }
 
+/**
+ * Bound for a `notice` summary. The account rides a collapsed transcript row
+ * and is committed to the durable log, while its inputs — task labels, goal
+ * objectives, tool arguments — are caller text with no length of their own.
+ */
+export const CONTEXT_SUMMARY_MAX_CHARS = 120
+
+/**
+ * Bound one `notice` summary to {@link CONTEXT_SUMMARY_MAX_CHARS}.
+ * @param summary - the producer's one-line account, of any length.
+ * @returns the account, ellipsized when it exceeds the bound.
+ */
+export function boundContextSummary(summary: string): string {
+  return summary.length <= CONTEXT_SUMMARY_MAX_CHARS
+    ? summary
+    : `${summary.slice(0, CONTEXT_SUMMARY_MAX_CHARS - 1)}…`
+}
+
 /** Any known message source, derived from {@link MessageSourceMap}; switch on `kind` and fall through unknowns (merge-extensible). */
 export type MessageSource = MessageSourceMap[keyof MessageSourceMap]