feat(workspace): a session in its own worktree, Claude's way

Desktop-parity phase 6, following Claude Desktop:
- the workspace home's Worktree checkbox starts the pane with
  `claude --worktree=<name>`, so Claude Code makes, populates
  (.worktreeinclude, WorktreeCreate hook) and cleans up
  <repo>/.claude/worktrees/<name>; AgentHub only excludes
  /.claude/worktrees/ at the repo root (Claude does not)
- server/lib/session-worktree.js resolves a session's worktree from the
  session row only (owned by the workspace, registered by git worktree
  list); every lane git route takes ?session= and answers ENOWORKTREE
  instead of falling back to the main folder
- the session view's Changes panel works in the opened run's worktree
  (dashboard_runs.worktree, GitTarget in the client api)
- Resume/Fork start in the session's worktree; a gone one is ENOWORKTREE
- Archive of a worktree session removes the worktree after asking:
  git worktree remove without --force, git branch -d (unmerged work keeps
  its branch), a lock held by a live pid refused, a stale one cleared
- listWorktrees reads `locked <reason>` locks
- tests scrub GIT_* env so the pre-commit suite cannot act on this repo

client/src/components/lanes/__tests__/WorkspaceHome.test.tsx

@@ -193,6 +193,21 @@ describe("WorkspaceHome", () => {
     );
   });
 
+  it("starts the session in its own worktree when Worktree is ticked (a repo only)", async () => {
+    const user = userEvent.setup();
+    renderHome();
+    await screen.findByTestId("workspace-recent-s-live");
+    await user.click(await screen.findByRole("checkbox", { name: "Worktree" }));
+    await user.type(screen.getByLabelText("How can I help you today?"), "try it");
+    await user.click(screen.getByTestId("workspace-home-start"));
+    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", {
+      slot: 1,
+      prompt: "try it",
+      permissionMode: "auto",
+      worktree: true,
+    });
+  });
+
   it("refuses to start when all four panes are busy", async () => {
     runs = [0, 1, 2, 3].map(
       (slot) => ({ id: `r${slot}`, laneId: 1, slot, status: "running" }) as RunHandle