feat(app): make the side panel user-directed (#3605)

* feat(app): let the file panel close its tree rail

The file panel's tree rail was permanent on desktop — the changes panel
had a folder-tree toggle in its toolbar, the file toolbar had nothing, so
the rail was there whether you wanted it or not.

The changes panel's private DesktopTreeToggle becomes a shared
TreeRailToggle so both toolbars render the same affordance; that also
drops the treeToggleStyle prop threaded through ChangesToolbarProps,
which recomputed exactly what the component already built internally.
The i18n keys move out of workspace.git.diff into workspace.tree, since a
shared component reaching into the git namespace would rot.

The two rails keep independent flags — the file rail in panel-store, the
changes rail in useChangesPreferences — so closing one leaves the other
alone. FileTreeToggle wires itself to the store rather than taking a
callback: the bar renders several components below FilePanel, where the
rail actually lives.

Both rails also get narrower (320 -> 260 default, 200 -> 180 minimum).
The persist version bump resets treeRailWidth on purpose: nearly every
existing install carries the old 320 default, so without the reset the
narrower rail would only ever reach new users. Anyone who dragged their
rail to a deliberate custom width loses it once.

Note that MIN_CONTENT_WIDTH still clamps the rail to
`containerWidth - 240`, so in a narrow pane the new default never
applies — that floor, not the default, is what sets the width there.

* feat(app): make the side panel an empty workspace surface

The fixed, hideable side panel keeps supporting context beside an agent without seeding content. Pane-local choices stay local, while implicit supporting opens follow a persisted routing preference.

* test(app): exercise side panel policy through its capability boundary

The browser rail journey already covers the real tree-toggle behavior. Side-panel policy tests now pass their platform capability through the public boundary instead of replacing a production module.

* test(app): expect an empty side panel after upgrade

* test(app): expect an empty first split pane

* test(app): account for the empty side panel in split retention

* feat(app): make workspace panel tabs independent

Give explicit New actions fresh tab instances, keep panel state opaque to tab management, preserve File navigation context, and stabilize uncached File transitions.

* fix(app): preserve file conflicts during preview refresh

Keep the current preview mounted for same-target reads and conversions so the editor retains its conflict observation state. Clear retained previews only when the file target changes.

packages/app/src/panels/agent-tracks.tsx

@@ -3,8 +3,8 @@ import { AgentTaskList } from "@/composer/task-list";
 import { ComposerTrackBar } from "@/composer/tracks";
 import { supportsDesktopPaneSplits, useIsCompactFormFactor } from "@/constants/layout";
 import { usePaneContext } from "@/panels/pane-context";
+import { useSettings } from "@/hooks/use-settings";
 import { useSessionStore } from "@/stores/session-store";
-import { useWorkspaceLayoutStore } from "@/stores/workspace-layout-store";
 import {
   type ArchiveFinishedStatus,
   useArchiveSubagent,
@@ -15,6 +15,7 @@ import { SubagentsTrack } from "@/subagents/track";
 import type { TodoEntry } from "@/types/stream";
 import { navigateToAgent } from "@/utils/navigate-to-agent";
 import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
+import { openSupportingTab } from "@/workspace-tabs/side-panel";
 
 /**
  * The pane's trackers — its subagents and its task list — as a row of pills above the composer.
@@ -39,6 +40,9 @@ export const AgentTracks = memo(function AgentTracks({
   const { workspaceId, tabId, openTab } = usePaneContext();
   const isCompact = useIsCompactFormFactor();
   const canSplit = supportsDesktopPaneSplits() && !isCompact;
+  const openInSidePanelByDefault = useSettings(
+    (settings) => settings.openSupportingTabsInSidePanel,
+  );
   const workspaceKey = buildWorkspaceTabPersistenceKey({ serverId, workspaceId });
   const canDetachSubagents = useSessionStore(
     (state) => state.sessions[serverId]?.serverInfo?.features?.agentDetach === true,
@@ -54,28 +58,34 @@ export const AgentTracks = memo(function AgentTracks({
         return;
       }
       if (canSplit && workspaceKey) {
-        useWorkspaceLayoutStore.getState().openTabInExplorerPaneFocused(workspaceKey, {
+        openSupportingTab({
+          isCompact,
+          workspaceKey,
           target: { kind: "agent", agentId: subagentId },
+          openInSidePanelByDefault,
           parentTabId: tabId,
         });
         return;
       }
       navigateToAgent({ serverId, agentId: subagentId });
     },
-    [canSplit, serverId, tabId, workspaceId, workspaceKey],
+    [canSplit, isCompact, openInSidePanelByDefault, serverId, tabId, workspaceId, workspaceKey],
   );
   const handleOpenProviderSubagent = useCallback(
     (parentAgentId: string, subagentId: string) => {
       if (canSplit && workspaceKey) {
-        useWorkspaceLayoutStore.getState().openTabInExplorerPaneFocused(workspaceKey, {
+        openSupportingTab({
+          isCompact,
+          workspaceKey,
           target: { kind: "provider_subagent", parentAgentId, subagentId },
+          openInSidePanelByDefault,
           parentTabId: tabId,
         });
         return;
       }
       openTab({ kind: "provider_subagent", parentAgentId, subagentId });
     },
-    [canSplit, openTab, tabId, workspaceKey],
+    [canSplit, isCompact, openInSidePanelByDefault, openTab, tabId, workspaceKey],
   );
 
   if (!hasAgentTracks({ subagentRows, tasks, archiveFinishedStatus })) {