agent term: add a setting to hide debug-level output

hyperhive#4008, mara: 'add a setting to not display verbose output...
like the grey colored debug stuff.'

Same shape as the existing expand-tool-output preference
(ExpandDetailsSetting.tsx): a new shared/src/prefs.ts key pair
(getHideDebugPref/setHideDebugPref), a new settings-menu-row component
owning its own state (HideDebugSetting.tsx, not a prop threaded through
the shared SettingsMenu component — per mara's earlier review on the
first one, more per-page options as props there is how that component
accumulates cruft), mounted next to ExpandDetailsSetting in Root.tsx.

Row.tsx skips (returns null for, not CSS display:none) any TermMsg
whose level is 'debug' when the pref is set — matches the muted 'debug'
row this issue is about (see docs/web-ui/terminal-rendering.md's Levels
table). Read live per-row, same as expand-details, so toggling applies
to newly streamed rows in an already-open tab without a reload; already
-rendered rows are unaffected either way, same non-retroactive
precedent the existing preference already sets.

frontend/packages/shared/src/prefs.ts

@@ -2,8 +2,9 @@
 // so a future second consumer (e.g. swarm-ui's own terminal) gets the
 // exact same key name for free via the browser's shared per-origin
 // storage, rather than an independent copy that could drift.
-// `ExpandDetailsSetting` (the per-agent page's own settings popover) is
-// the only writer today; `Row.tsx` is the only reader.
+// `ExpandDetailsSetting` / `HideDebugSetting` (the per-agent page's own
+// settings popover rows) are the only writers today; `Row.tsx` is the
+// only reader of either.
 
 const EXPAND_DETAILS_KEY = "hive-agent-expand-details";
 
@@ -29,3 +30,27 @@ export function setExpandDetailsPref(v: boolean): void {
     /* localStorage unavailable — preference is session-only */
   }
 }
+
+const HIDE_DEBUG_KEY = "hive-agent-hide-debug";
+
+// Whether a per-agent terminal's `debug`-level rows (thinking, coalesced
+// ticks, ambient harness chatter — see docs/web-ui/terminal-rendering.md's
+// Levels table) should be skipped entirely rather than rendered muted.
+// Same shape as `getExpandDetailsPref` above: pure client-side, no
+// backend field, read live (not cached) so a change applies to the next
+// rendered row without a reload.
+export function getHideDebugPref(): boolean {
+  try {
+    return localStorage.getItem(HIDE_DEBUG_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+export function setHideDebugPref(v: boolean): void {
+  try {
+    if (v) localStorage.setItem(HIDE_DEBUG_KEY, "1");
+    else localStorage.removeItem(HIDE_DEBUG_KEY);
+  } catch {
+    /* localStorage unavailable — preference is session-only */
+  }
+}