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/goal/tool-goal/src/index.ts

@@ -315,7 +315,12 @@ export function apply(ctx: Context, config: Config): void {
           content: args.action === 'complete'
             ? renderWrapupContext(goal.objective)
             : renderWrapupContext(goal.objective, args.blocked_reason as string),
-          source: { kind: 'plugin', plugin: 'tool-goal' },
+          source: {
+            kind: 'plugin',
+            plugin: 'tool-goal',
+            form: 'notice',
+            summary: `${args.action as string}: ${goal.objective}`,
+          },
         }))
       }
       return Promise.resolve(goalValue(goal))

packages/goal/tool-goal/tests/tool-goal.spec.ts

@@ -372,7 +372,12 @@ describe('goal tool state transitions', () => {
     expect(complete.concludesTurn).toBeUndefined()
     const contexts = complete.additionalContexts ?? []
     expect(contexts).toHaveLength(1)
-    expect(contexts[0]?.source).toEqual({ kind: 'plugin', plugin: 'tool-goal' })
+    expect(contexts[0]?.source).toEqual({
+      kind: 'plugin',
+      plugin: 'tool-goal',
+      form: 'notice',
+      summary: 'complete: pause cleanly',
+    })
     const block = contexts[0]?.content[0]
     if (block?.type !== 'text') throw new Error('expected one text wrap-up block')
     expect(block.text).toContain('<goal_complete>')