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>

ui/app.css

@@ -1614,6 +1614,21 @@ details[open] > summary .step__chev .icon { transform: rotate(90deg); }
 }
 .termpanel__last:empty { display: none; }
 
+/* --- The subagent tree (spec 3.4.0) ------------------------------------------------------------
+   Rows are the tasks' rows (.agent): name and id on one line, the position, then the phase (or how
+   it ended) clipped to one line -- its clock is in the opened row. Ended ones are dimmed. */
+
+.subagents { display: grid; gap: var(--sp-2); min-width: 0; }
+.subagents .tasks__team, .subagents .tasks__goal { margin: 0; }
+.subagent__who { display: flex; gap: var(--sp-2); align-items: baseline; min-width: 0; }
+.subagent__who .agent__id { flex: none; }
+.subagent__phase { overflow: hidden; white-space: nowrap; text-overflow: ellipsis; font-size: var(--text-xs); color: var(--ink-muted); }
+.agent.is-ended { opacity: var(--opacity-dim); }
+.subagent__buttons { display: flex; flex-wrap: wrap; gap: var(--sp-2); margin: 0; }
+.findings { margin: 0; padding: var(--sp-3) var(--sp-3) var(--sp-3) calc(var(--sp-3) + var(--sp-5)); display: grid; gap: var(--sp-3); }
+.findings__item { min-width: 0; overflow-wrap: anywhere; }
+.ask__subagent { color: var(--ink); }
+
 /* --- Narrower windows ---------------------------------------------------------------------- */
 
 /* 64rem (--bp-wide): a panel goes over the chat instead of beside it. */