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/Workspace.tsx

@@ -493,6 +493,11 @@ export function Workspace() {
     });
   }, []);
 
+  // The session the opened run holds, when that run works in a worktree of
+  // its own (`claude --worktree`): the Changes panel shows that tree.
+  const openedRun = runParam ? activeRuns?.items.find((r) => r.id === runParam) : undefined;
+  const gitSession = openedRun?.worktree && openedRun.sessionId ? openedRun.sessionId : undefined;
+
   // A pane the layout does not show yet (`&pane=1` on one pane) widens the
   // split so the run lands where it was asked for — once per request, so the
   // user can still go back to one pane afterwards.
@@ -1065,7 +1070,11 @@ export function Workspace() {
                   </button>
                 </div>
                 <LaneChangesPanel
+                  // A new tree is a new panel: no read for the previous one
+                  // can land in it.
+                  key={`${currentLane.id}:${gitSession ?? ""}`}
                   laneId={currentLane.id}
+                  session={gitSession}
                   tab={panelTab}
                   full={changesSize === "full"}
                   // A sibling worktree forks from the lane's SOURCE repo when