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/repeat-tool-guard/session.jsonl

@@ -2,7 +2,7 @@
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Write the todo list 'watch the kettle boil' five times in a row without changing it, then reply DONE."}],"source":{"kind":"user"},"role":"user","id":"0a8a60c7-eece-48d5-ac75-fb39c9d79c56"},"surfaceOp":"append"}
 {"type":"session/title","seq":2,"time":0,"data":{"title":"Write the todo list 'watch","messageSeqs":[1],"source":{"kind":"fallback"}}}
-{"type":"user/message","seq":3,"time":1785464649934,"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":"60a180b9-4aea-4d04-87aa-9e19cde4d6de"},"surfaceOp":"append"}
+{"type":"user/message","seq":3,"time":1785464649934,"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":"60a180b9-4aea-4d04-87aa-9e19cde4d6de"},"surfaceOp":"append"}
 {"type":"step/start","seq":4,"time":1785464649934,"data":{"turn":1,"step":1}}
 {"type":"request/header","seq":5,"time":1785464649935,"data":{"header":{"config":{"provider":"deepseek-official","model":"deepseek-v4-flash"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
 {"type":"request/context","seq":6,"time":1785487607439,"data":{"provider":"deepseek-official","model":"deepseek-v4-flash"}}
@@ -37,7 +37,7 @@
 {"type":"tool/call","seq":35,"time":1785487607491,"data":{"turn":1,"step":3,"callId":"call_3","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"watch the kettle boil\", \"status\": \"in_progress\"}]}"}}
 {"type":"todo/write","seq":36,"time":1785487607499,"data":{"todos":[{"content":"watch the kettle boil","status":"in_progress"}]}}
 {"type":"tool/result","seq":37,"time":1785487607499,"data":{"turn":1,"step":3,"message":{"source":{"kind":"tool","callId":"call_3"},"content":[{"type":"tool-result","toolCallId":"call_3","content":[{"type":"text","text":"Updated todo list: 0 pending, 1 in progress, 0 completed."}],"isError":false}],"role":"user","id":"837f2606-2c17-44b5-b795-406f48efab66"}},"sourceEventSeqs":[35],"surfaceOp":"append"}
-{"type":"user/message","seq":38,"time":1785487607499,"data":{"content":[{"type":"text","text":"You are repeating the exact same tool call with identical arguments. Carefully analyze the previous result before calling again: if the task is not complete, try a different approach or different arguments instead of repeating the call."}],"source":{"kind":"plugin","plugin":"repeat-tool-guard"},"role":"user","id":"6d850e11-d21a-4688-b6ca-abcf83f5c536"},"surfaceOp":"append"}
+{"type":"user/message","seq":38,"time":1785487607499,"data":{"content":[{"type":"text","text":"You are repeating the exact same tool call with identical arguments. Carefully analyze the previous result before calling again: if the task is not complete, try a different approach or different arguments instead of repeating the call."}],"source":{"kind":"plugin","plugin":"repeat-tool-guard","form":"notice","summary":"todo_write × 3"},"role":"user","id":"6d850e11-d21a-4688-b6ca-abcf83f5c536"},"surfaceOp":"append"}
 {"type":"step/end","seq":39,"time":1785487607499,"data":{"turn":1,"step":3}}
 {"type":"step/start","seq":40,"time":1785487607506,"data":{"turn":1,"step":4}}
 {"type":"assistant/chunk","seq":41,"time":0,"data":{"turn":1,"step":4,"chunk":{"type":"block-start","index":0,"blockType":"tool-call"}}}
@@ -60,7 +60,7 @@
 {"type":"tool/call","seq":58,"time":1785487607531,"data":{"turn":1,"step":5,"callId":"call_5","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"watch the kettle boil\", \"status\": \"in_progress\"}]}"}}
 {"type":"todo/write","seq":59,"time":1785487607538,"data":{"todos":[{"content":"watch the kettle boil","status":"in_progress"}]}}
 {"type":"tool/result","seq":60,"time":1785487607538,"data":{"turn":1,"step":5,"message":{"source":{"kind":"tool","callId":"call_5"},"content":[{"type":"tool-result","toolCallId":"call_5","content":[{"type":"text","text":"Updated todo list: 0 pending, 1 in progress, 0 completed."}],"isError":false}],"role":"user","id":"5f2668c7-6161-45f5-94e0-3dee26391efa"}},"sourceEventSeqs":[58],"surfaceOp":"append"}
-{"type":"user/message","seq":61,"time":1785487607538,"data":{"content":[{"type":"text","text":"Repeated tool call detected:\n- tool: todo_write\n- consecutive_calls: 5\n- arguments: {\"todos\":[{\"content\":\"watch the kettle boil\",\"status\":\"in_progress\"}]}\nThe repeated calls are not making progress. Do not call this tool with these exact arguments again. Inspect the latest result and choose a different action, different arguments, or finish the task if enough evidence has been gathered."}],"source":{"kind":"plugin","plugin":"repeat-tool-guard"},"role":"user","id":"55ed1387-137f-4fe9-b149-17c2d781874c"},"surfaceOp":"append"}
+{"type":"user/message","seq":61,"time":1785487607538,"data":{"content":[{"type":"text","text":"Repeated tool call detected:\n- tool: todo_write\n- consecutive_calls: 5\n- arguments: {\"todos\":[{\"content\":\"watch the kettle boil\",\"status\":\"in_progress\"}]}\nThe repeated calls are not making progress. Do not call this tool with these exact arguments again. Inspect the latest result and choose a different action, different arguments, or finish the task if enough evidence has been gathered."}],"source":{"kind":"plugin","plugin":"repeat-tool-guard","form":"notice","summary":"todo_write × 5"},"role":"user","id":"55ed1387-137f-4fe9-b149-17c2d781874c"},"surfaceOp":"append"}
 {"type":"step/end","seq":62,"time":1785487607538,"data":{"turn":1,"step":5}}
 {"type":"step/start","seq":63,"time":1785487607545,"data":{"turn":1,"step":6}}
 {"type":"assistant/chunk","seq":64,"time":0,"data":{"turn":1,"step":6,"chunk":{"type":"block-start","index":0,"blockType":"text"}}}