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/utils/prepare-workspace-tab.ts

@@ -9,7 +9,11 @@ export interface PrepareWorkspaceTabInput {
 }
 
 export interface PrepareWorkspaceTabDeps {
-  openTabFocused: (workspaceKey: string, target: WorkspaceTabTarget) => string | null;
+  openTab: (input: {
+    workspaceKey: string;
+    target: WorkspaceTabTarget;
+    intent: "reveal";
+  }) => string | null;
   pinAgent: (workspaceKey: string, agentId: string) => void;
 }
 
@@ -31,7 +35,7 @@ export function prepareWorkspaceTab(
       workspaceId: input.workspaceId,
     }) ?? "";
 
-  deps.openTabFocused(key, target);
+  deps.openTab({ workspaceKey: key, target, intent: "reveal" });
 
   if (input.pin && target.kind === "agent") {
     deps.pinAgent(key, target.agentId);

packages/app/src/utils/workspace-navigation.test.ts

@@ -22,7 +22,13 @@ function createFakeLayout() {
   return {
     openedTabs,
     pinnedAgents,
-    openTabFocused: (key: string, target: WorkspaceTabTarget) => {
+    openTab: ({
+      workspaceKey: key,
+      target,
+    }: {
+      workspaceKey: string;
+      target: WorkspaceTabTarget;
+    }) => {
       openedTabs.push({ key, target });
       return target.kind === "agent" ? target.agentId : null;
     },

packages/app/src/utils/workspace-navigation.ts

@@ -3,13 +3,15 @@ import {
   prepareWorkspaceTab as prepareWorkspaceTabPure,
   type PrepareWorkspaceTabInput,
 } from "./prepare-workspace-tab";
+import type { WorkspaceTabTarget } from "@/workspace-tabs/model";
 
 export type { PrepareWorkspaceTabInput } from "./prepare-workspace-tab";
 
 function layoutStoreDeps() {
   const store = useWorkspaceLayoutStore.getState();
   return {
-    openTabFocused: store.openTabFocused,
+    openTab: (input: { workspaceKey: string; target: WorkspaceTabTarget; intent: "reveal" }) =>
+      store.openTab(input),
     pinAgent: store.pinAgent,
   };
 }