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/Checkbox.tsx

@@ -78,6 +78,8 @@ export interface CheckboxProps {
    *  (bypass permissions, purge data, force). */
   tone?: "accent" | "danger" | "warning";
   disabled?: boolean;
+  /** Tooltip (the app turns every `title` into its themed bubble). */
+  title?: string;
 }
 
 const TONE = {
@@ -99,6 +101,7 @@ export function Checkbox({
   labelClassName,
   tone = "accent",
   disabled = false,
+  title,
 }: CheckboxProps) {
   return (
     <button
@@ -106,6 +109,7 @@ export function Checkbox({
       role="checkbox"
       aria-checked={checked}
       disabled={disabled}
+      title={title}
       onClick={() => onChange(!checked)}
       className={`group inline-flex items-center gap-2 text-left disabled:cursor-not-allowed disabled:opacity-50 focus-visible:outline-none ${className ?? ""}`}
     >