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/tests/tool-tasks.spec.ts

@@ -460,7 +460,12 @@ describe('completion notices', () => {
       id: expect.any(String) as unknown,
       role: 'user',
       content: [{ type: 'text', text: 'background task bash-1 (bash: pnpm test) finished [status: completed, exit code: 0]. Read its output with task_output.' }],
-      source: { kind: 'plugin', plugin: 'tool-tasks' },
+      source: {
+        kind: 'plugin',
+        plugin: 'tool-tasks',
+        form: 'notice',
+        summary: 'bash pnpm test [status: completed, exit code: 0]',
+      },
     })
   })
 
@@ -484,7 +489,14 @@ describe('completion notices', () => {
         id: expect.any(String) as unknown,
         role: 'user',
         content: [{ type: 'text', text: 'background task subagent-1\n[notice truncated]\nDone; task_output.' }],
-        source: { kind: 'plugin', plugin: 'tool-tasks' },
+        // The label and status detail are unbounded caller text, so the durable
+        // one-line account caps itself rather than committing their full length.
+        source: {
+          kind: 'plugin',
+          plugin: 'tool-tasks',
+          form: 'notice',
+          summary: `subagent ${'x'.repeat(110)}…`,
+        },
       },
     )