agent web UI: add terminal verbosity setting (expand tool output by default)

Adds a browser-local (localStorage only, no backend field) toggle in
the per-agent overflow menu's new settings section: whether otherwise-
collapsed <details> rows in the live terminal (long tool-results,
Write/Edit diffs, ...) default open. Message-bearing rows that already
default open (send/ask/answer/recv) are unaffected either way.

The shared terminal factory (frontend/packages/shared/src/terminal/terminal.js)
gains an optional expandDetails option (boolean or zero-arg function),
read live on every details()/detailsDiff() call rather than captured
once, so flipping the toggle mid-session applies to the next rendered
row without a reload. Unused by the dashboard's own terminal pane, so
its default-closed behaviour is unchanged.

Closes #2961.

docs/web-ui/agent.md

@@ -101,6 +101,16 @@ through. Three flex columns:
     the `/effort <level>` slash command). The active level's button is
     highlighted; `renderEffortChip` keeps the picker in sync with
     `StateSnapshot.effort` from the cold-load snapshot.
+    A third separator + **settings** section holds a single
+    `role="menuitemcheckbox"` toggle, `☐/☑ expand tool output` — flips
+    whether otherwise-collapsed `<details>` rows in the terminal (long
+    tool-results, Write/Edit diffs, …) default open. Pure client-side:
+    persisted to this browser's `localStorage` only (no backend field,
+    no `/api/*` call), read live by the shared terminal factory's
+    `expandDetails` option (see docs/web-ui/shape.md::Shared terminal
+    pane) so toggling mid-session applies to the next rendered row
+    without a reload. Rows that already default open regardless (send /
+    ask / answer / recv) are unaffected either way.
     The popover's display rules are scoped to `:not([hidden])` so the
     `[hidden]` HTML attribute's UA `display: none` isn't overridden by
     the author CSS's `display: flex` — the popover stays hidden until