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/core/system-prompt/src/index.ts

@@ -208,7 +208,19 @@ export function renderPrompt(assembly: PromptAssembly): string {
  * @returns the current full snapshot, or `''` when no context is active.
  */
 export function renderContextSnapshot(assembly: PromptAssembly): string {
-  const body = renderContextSections(assembly).map(section => section.text).join('\n\n')
+  return joinContextSections(renderContextSections(assembly))
+}
+
+/**
+ * The model-facing snapshot text for an already-rendered section list.
+ *
+ * A caller that also needs the sections renders them once and joins here, so a
+ * request does not interpolate every context twice.
+ * @param sections - sections from {@link renderContextSections}.
+ * @returns the current full snapshot, or `''` when no context is active.
+ */
+export function joinContextSections(sections: readonly ContextSnapshotSection[]): string {
+  const body = sections.map(section => section.text).join('\n\n')
   if (body.length === 0) return ''
   return `Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\n${body}`
 }