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/subagent/subagent/src/continuation.ts

@@ -47,6 +47,8 @@ import type SubagentActivationSetupRegistry from './activation-setup-registry.ts
 /** Attribution for a model coordinator's follow-up to one of its children. */
 export 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
 }
@@ -54,6 +56,8 @@ export interface CoordinatorMessageSource {
 /** Durable attribution for a continuable child's explicit parent report. */
 export 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
 }
@@ -481,6 +485,7 @@ export class SubagentContinuationManager {
       ],
       source: {
         kind: 'subagent-report' as const,
+        form: 'relay' as const,
         senderSessionId: activation.childId,
       },
     })

packages/subagent/tool-subagent-control/src/index.ts

@@ -66,7 +66,7 @@ export function apply(ctx: Context): void {
         SessionId(args.subagent_id),
         message,
         {
-          source: { kind: 'coordinator', senderSessionId: parent.id },
+          source: { kind: 'coordinator', form: 'relay', senderSessionId: parent.id },
           signal: exec.signal,
         },
       )

packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts

@@ -102,6 +102,7 @@ describe('dsh-tool-subagent-control', () => {
     // Durable provenance records the calling agent without granting authority.
     expect(followUp?.type === 'user/message' && followUp.data.source).toEqual({
       kind: 'coordinator',
+      form: 'relay',
       senderSessionId: parent.id,
     })
   })