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/LaneGitBar.tsx

@@ -36,7 +36,7 @@ import {
   Trash2,
   Upload,
 } from "lucide-react";
-import { api, type GitAuthNeeded } from "../../lib/api";
+import { api, gitTarget, type GitAuthNeeded } from "../../lib/api";
 import type { LaneGitFacts, LaneWorktree } from "../../lib/types";
 
 type Action =
@@ -89,12 +89,15 @@ function Btn({
 
 export function LaneGitBar({
   laneId,
+  session,
   git,
   onChanged,
   onNewLane,
   lead,
 }: {
   laneId: number;
+  /** Work in that session's own worktree (see `gitTarget`). */
+  session?: string;
   git: LaneGitFacts;
   /** Rendered at the start of the action row: the header's branch line, so
    *  the facts and the buttons that act on them read as one row. */
@@ -157,7 +160,7 @@ export function LaneGitBar({
     setBusy(action);
     setError(null);
     try {
-      const res = await api.lanes.gitAction(lane, action, body || {});
+      const res = await api.lanes.gitAction(gitTarget(lane, session), action, body || {});
       // Landed after a lane switch: every write below belongs to the lane
       // no longer shown.
       if (laneNow.current !== lane) return;
@@ -208,7 +211,7 @@ export function LaneGitBar({
 
   const refreshBranches = async () => {
     try {
-      const r = await api.lanes.gitBranches(laneId);
+      const r = await api.lanes.gitBranches(gitTarget(laneId, session));
       if (laneNow.current !== laneId) return;
       setBranches(r.branches);
       setWorktrees(r.worktrees || []);