Subagents (C4): the tree in the activity panel, read-only sessions, findings, stop; agent chats' setting

On the server's /api/v2 (3.4.0, server dev 5d7b79c):
- The activity panel reads GET /chats/{chat}/tree beside the activity and
  draws ui/js/tree.js's rows in a Subagents section, the server panel's words
  and layout: nested by parent, position, phase, context %, state, model,
  mode + request options, messages and the last one, open slots, the ended
  ones folded and dimmed. A team's members are no longer drawn again among
  the tasks. The stream's `tree` frame redraws it.
- A subagent's conversation (/subagents/{id}/transcript, read again every 3 s
  while it works) and its findings open read-only in the panel; Stop asks,
  then POSTs /stop. With an agent chat's device away: the kept tree with the
  server's note, and its 409 sentence for the rest.
- A new agent chat's Subagents chip from /models?kind=agent&device= (each
  model's `subagents`, `agent_subagents.offered` and `device_runs`); an open
  agent chat's from its own `subagents`, as before.
- An approval item's `subagent` names it on the card.
- Progress keys are the spec's own ({{field}}), as the server's catalogues
  have them; SERVER_PENDING is gone, every sentence is translated.

tests/ui reads the server's contract fixtures (scripts/fetch-fixtures.sh ->
tests/ui/fixtures, SOURCE says the commit): status frames, tree, tree frame,
transcript, findings, models?kind=agent, a subagent's approval; six C4 cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ui/fixtures/event_ask.json

@@ -0,0 +1,162 @@
+{
+  "cards": [
+    {
+      "id": "card1",
+      "kind": "question",
+      "why": "",
+      "dismissable": false,
+      "expires_in": null,
+      "items": [
+        {
+          "index": 0,
+          "key": "q0",
+          "kind": "question",
+          "tool": "ask_user",
+          "title": "Honey or not?",
+          "header": "Taste",
+          "detail": "",
+          "reason": "",
+          "purpose": "",
+          "options": [
+            {
+              "label": "Honey",
+              "description": "",
+              "recommended": true
+            },
+            {
+              "label": "Plain",
+              "description": "As the elves have it",
+              "recommended": false
+            }
+          ],
+          "multiple": false,
+          "free_text": true,
+          "editable": false,
+          "plan": null,
+          "mode_choice": true,
+          "job": "",
+          "subagent": null
+        }
+      ]
+    },
+    {
+      "id": "card2",
+      "kind": "approval",
+      "why": "",
+      "dismissable": false,
+      "expires_in": null,
+      "items": [
+        {
+          "index": 0,
+          "key": "0",
+          "kind": "approval",
+          "tool": "bash",
+          "title": "Run a command",
+          "header": "",
+          "detail": "rm -rf build",
+          "reason": "Edit mode asks before anything that runs a command.",
+          "purpose": "Start clean",
+          "options": [],
+          "multiple": false,
+          "free_text": false,
+          "editable": true,
+          "plan": null,
+          "mode_choice": true,
+          "job": "explore (job1)",
+          "subagent": null
+        }
+      ]
+    },
+    {
+      "id": "card3",
+      "kind": "plan",
+      "why": "",
+      "dismissable": true,
+      "expires_in": null,
+      "items": [
+        {
+          "index": 0,
+          "key": "p0",
+          "kind": "plan",
+          "tool": "plan_submit",
+          "title": "Bake lembas",
+          "header": "",
+          "detail": "",
+          "reason": "",
+          "purpose": "",
+          "options": [],
+          "multiple": false,
+          "free_text": true,
+          "editable": false,
+          "plan": {
+            "version": 2,
+            "title": "Bake lembas",
+            "summary": "",
+            "findings": [],
+            "objectives": [],
+            "phases": [
+              {
+                "id": "p1",
+                "title": "Plan",
+                "status": "pending",
+                "tasks": [
+                  {
+                    "id": "t1",
+                    "text": "Gather mallorn leaves",
+                    "status": "todo",
+                    "note": ""
+                  },
+                  {
+                    "id": "t2",
+                    "text": "Bake slowly",
+                    "status": "todo",
+                    "note": ""
+                  }
+                ]
+              }
+            ],
+            "steps": [
+              "Gather mallorn leaves",
+              "Bake slowly"
+            ]
+          },
+          "mode_choice": true,
+          "job": "",
+          "subagent": null
+        }
+      ]
+    },
+    {
+      "id": "card4",
+      "kind": "approval",
+      "why": "",
+      "dismissable": false,
+      "expires_in": null,
+      "items": [
+        {
+          "index": 0,
+          "key": "0",
+          "kind": "approval",
+          "tool": "bash",
+          "title": "Run a command",
+          "header": "",
+          "detail": "git push",
+          "reason": "Edit mode asks before anything that runs a command.",
+          "purpose": "",
+          "options": [],
+          "multiple": false,
+          "free_text": false,
+          "editable": true,
+          "plan": null,
+          "mode_choice": true,
+          "job": "",
+          "subagent": {
+            "id": "s1",
+            "name": "parser",
+            "position": "reviews the parser"
+          }
+        }
+      ]
+    }
+  ]
+}