feat(files): follow the agent into a worktree outside the session folder

#772 made the Files panel follow an agent into a worktree inside the
session's folder. A worktree beside the repository (git worktree add
../feature, or an EnterWorktree into one that already exists there)
stayed unfollowed, since serving it needs a second confinement root.

findOutsideWorktree() answers that case without running git. The
nearest .git above the pane must name <common dir>/worktrees/<id> of the
session's own repository, and that registry entry must name the
candidate back (or the .git folder must be the common dir itself). It
refuses /, the home folder and above, any folder holding workingDir and
Docker case workspaces, and multi-user mode applies the owner's
folderAllowedForUser rule. The answer is liveWorktree with outside: true
and the folder's real path.

The file routes serve an absolute path inside that worktree, and only
while the folder still resolves to the recorded path. A relative path
still resolves against workingDir alone. The browser routes such paths
to the editable preview, and root-relative markdown refs resolve from
the top of the worktree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/session.ts

@@ -1639,10 +1639,11 @@ export class Session extends EventEmitter {
   }
 
   /**
-   * The linked git worktree inside `workingDir` that {@link paneCwd} is in, when there is one
-   * (`live-worktree.ts`). The Files panel lists it instead of the main checkout. Runtime-only and
-   * never a confinement root, like `paneCwd`: it is judged lexically when the pane moves, and the
-   * listing route re-checks the real folder on every request.
+   * The linked git worktree that {@link paneCwd} is in, when there is one (`live-worktree.ts`).
+   * The Files panel lists it instead of the main checkout. Runtime-only, like `paneCwd`. One inside
+   * `workingDir` is never a confinement root: it is judged lexically when the pane moves, and the
+   * listing route re-checks the real folder on every request. One outside it (`outside: true`) is
+   * a second root for the file routes, re-checked on every request the same way.
    */
   get liveWorktree(): LiveWorktree | undefined {
     return this._liveWorktree;