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/pages/__tests__/Workspace.test.tsx

@@ -722,6 +722,34 @@ describe("Workspace — home and session view", () => {
     await userEvent.setup().click(screen.getByTestId("workspace-back-home"));
     expect(await screen.findByTestId("workspace-home")).toBeInTheDocument();
   });
+  it("points the Changes panel at the opened run's worktree session", async () => {
+    localStorage.setItem("agenthub.workspace.changesOpen", "1");
+    const original = vi.mocked(api.run.list).getMockImplementation();
+    vi.mocked(api.run.list).mockResolvedValue({
+      runs: [],
+      items: [
+        {
+          id: "agenthub-lane-1-1",
+          laneId: 1,
+          slot: 1,
+          status: "running",
+          sessionId: "s-wt",
+          worktree: true,
+        },
+      ],
+    } as never);
+    render(
+      <MemoryRouter initialEntries={["/workspace?lane=1&run=agenthub-lane-1-1"]}>
+        <Workspace />
+      </MemoryRouter>
+    );
+    await settle();
+    await settle();
+    await waitFor(() => expect(api.lanes.git).toHaveBeenCalledWith({ lane: 1, session: "s-wt" }));
+    vi.mocked(api.run.list).mockImplementation(original!);
+    localStorage.removeItem("agenthub.workspace.changesOpen");
+  });
+
   it("widens one pane to two for &pane=1, so the run opens beside the first", async () => {
     localStorage.removeItem("agenthub.workspace.splitView");
     render(