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.

examples/acp-agent/tests/snapshots/subagent-report/session.jsonl

@@ -2,7 +2,7 @@
 {"type":"turn/start","seq":0,"time":1789000000001,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1789000000002,"data":{"content":[{"type":"text","text":"Follow these steps exactly, then stop. 1. Call the subagent tool once with run_in_background set to true, description 'Report a finding', and prompt 'Call the report tool once with output exactly CHILD_REPORT_OK, then stop.'. 2. Reply with the single word STARTED. Do not use the bash tool."}],"source":{"kind":"user"},"role":"user","id":"dd8fabed-440b-4993-a5b3-c8dc8276b5bc"},"surfaceOp":"append"}
 {"type":"session/title","seq":2,"time":1789000000002,"data":{"title":"Follow these steps exactly, then","messageSeqs":[1],"source":{"kind":"fallback"}}}
-{"type":"user/message","seq":3,"time":1785501592842,"data":{"content":[{"type":"text","text":"Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nCurrent DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations.\n\nApproval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."}],"source":{"kind":"plugin","plugin":"@deepseek-ai/dsh-system-prompt"},"role":"user","id":"883e0516-bfc1-4036-bcb1-65e4bfec065d"},"surfaceOp":"append"}
+{"type":"user/message","seq":3,"time":1785501592842,"data":{"content":[{"type":"text","text":"Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nCurrent DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations.\n\nApproval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."}],"source":{"kind":"plugin","plugin":"@deepseek-ai/dsh-system-prompt","form":"snapshot","sections":[{"name":"sandbox:policy","text":"Current DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations."},{"name":"approval:policy","text":"Approval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."}]},"role":"user","id":"883e0516-bfc1-4036-bcb1-65e4bfec065d"},"surfaceOp":"append"}
 {"type":"step/start","seq":4,"time":1785501592842,"data":{"turn":1,"step":1}}
 {"type":"request/header","seq":5,"time":1785501592842,"data":{"header":{"config":{"provider":"deepseek-official","model":"deepseek-v4-flash"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
 {"type":"request/context","seq":6,"time":1785501592843,"data":{"provider":"deepseek-official","model":"deepseek-v4-flash"}}
@@ -24,7 +24,7 @@
 {"type":"assistant/message","seq":22,"time":1785501592877,"data":{"turn":1,"step":2,"message":{"role":"assistant","content":[{"type":"text","text":"STARTED"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"603fbfa3-1810-495e-a640-7073392b496b"},"usage":{"inputTokens":10,"outputTokens":5}},"sourceEventSeqs":[17,18,19,20,21],"surfaceOp":"append"}
 {"type":"step/end","seq":23,"time":1785501592877,"data":{"turn":1,"step":2}}
 {"type":"turn/end","seq":24,"time":1785501592877,"data":{"turn":1,"reason":{"kind":"completed"}}}
-{"type":"user/message","seq":25,"time":1785469571237,"data":{"content":[{"type":"text","text":"Background subagent 33333333-3333-4333-8333-333333333333 reported:"},{"type":"text","text":"CHILD_REPORT_OK"}],"source":{"kind":"subagent-report","senderSessionId":"33333333-3333-4333-8333-333333333333"},"role":"user","id":"418727a1-5001-4735-ae60-2ca33256ac3f"},"surfaceOp":"append"}
+{"type":"user/message","seq":25,"time":1785469571237,"data":{"content":[{"type":"text","text":"Background subagent 33333333-3333-4333-8333-333333333333 reported:"},{"type":"text","text":"CHILD_REPORT_OK"}],"source":{"kind":"subagent-report","form":"relay","senderSessionId":"33333333-3333-4333-8333-333333333333"},"role":"user","id":"418727a1-5001-4735-ae60-2ca33256ac3f"},"surfaceOp":"append"}
 {"type":"turn/start","seq":26,"time":1785501592940,"data":{"turn":2,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":27,"time":1785501592940,"data":{"content":[{"type":"text","text":"Repeat back, verbatim, the exact output the background subagent reported to you. Reply with only that text. Do not use any tools."}],"source":{"kind":"user"},"role":"user","id":"c079dbd9-893d-42b3-a260-417e8de8adca"},"surfaceOp":"append"}
 {"type":"step/start","seq":28,"time":1785501592944,"data":{"turn":2,"step":1}}